Denise Fergus says her son James is still very much part of the family. A large portrait of him hangs on the living room wall, and her three younger boys regard him as their elder brother; they often talk about him. “James is never far from our minds. I brought the lads up knowing him, even though they never met him.”

It is 25 years since James Bulger was murdered, and it has taken Fergus this long to write about it. It’s not that she didn’t want to: it was just too painful. We meet at her publisher’s office, where she admits she has not been looking forward to this interview. Her second husband, Stuart, the father of her two youngest children, is here to support her.

James Bulger was killed one month short of his third birthday in 1993. It was a murder that shocked, and which continues to shock, Britain. James was tortured to death by two 10-year-olds, Robert Thompson and Jon Venables, who left him on a railway track to be hit by a train.

The CCTV footage of the blond toddler walking away from Liverpool’s Strand shopping centre, hand in hand with one of his killers, remains one of the most chilling images of the 20th century, not least because it looks so innocent. The murder and trial consumed the country. How could two young boys commit such a crime? Three days after James’s body was found, the then prime minister John Major talked about the Tories’ tough stance on crime, saying: “Society needs to condemn a little more and understand a little less.” And sure enough, society did condemn. When Thompson and Venables arrived at Preston crown court for their trial nine months later, in November 1993, it looked as if the baying mob would tear them to pieces.

The two boys were found guilty and sentenced to a minimum of eight years, making them Britain’s youngest murderers. In the wake of their trial, Fergus – who left school with no formal qualifications – took on the legal profession and the government, believing their sentence too lenient. It was increased to 15 years by the then home secretary, Michael Howard, but he was later overruled.

Thompson and Venables were released from secure children’s homes in 2001, after eight years. Since their release, Fergus has campaigned against their right to anonymity, and the money spent on those new identities.

Her new book, I Let Him Go, is part memoir and part excoriation of the legal system she says has let her down. But there is another reason she has written it – to bring her son back to life. She is tired of James being remembered simply as the little boy who was murdered. She wants us to know him as the livewire he was: “He was very bubbly. He loved dancing to Michael Jackson videos and making people laugh. My happiest memory of him is him running towards me with his hair bouncing everywhere. He didn’t walk anywhere. He’d run into your arms with a big smile on his face.” The memory of his smile makes her smile, transforming her: over the years, Fergus’s anger and grief have etched themselves into her face.

On both hands she wears multiple rings – three on each ring finger. Most striking is a gold signet ring that says MUM. On her coat is a heart-shaped For James pin badge, illustrated with the image of a little boy holding hands with a slightly older girl.

Her book is a painful, sometimes traumatic read. Fergus reveals she is haunted by the idea that, at the end, James was calling for her and she didn’t come. For many years, when she thought of his last few minutes, she had panic attacks and was unable to breathe.

But the most heartbreaking aspect of I Let Him Go is its title. Fergus was a cautious young mother, who nearly always took James out shopping in a buggy. But not that day: she let go of his hand in the butcher’s shop for a second to take out her purse, long enough for Thompson and Venables to snatch James. For so many years, she says, she would obsessively ask herself why she hadn’t taken the buggy, why she let go of his hand, why she turned left rather than right when she came out to look for him. In the book she writes: “When you’ve lost a child, you go through stages. You blame yourself. You blame others. But at the end of it there are only two people to blame in this, and that’s the two who took him. But it did take me a long time to realise that and get my head around it. It wasn’t me that killed James.”

The book’s title suggests you still blame yourself, I say. “Well, I do, because I was the last to see James alive.” She pauses, and quietly corrects herself. “One of the last.”

Fergus, now 50, still doesn’t know much about what happened that day. She did not attend the three-week trial (she was heavily pregnant with her second son, Michael, and was told the stress might make her miscarry). If there is news in the papers, Stuart goes through it with a thick black marker to blank out any details she would not want to know. Astonishingly, many of us know more about the murder of James Bulger than his mother does.

***

Denise Fergus grew up in a happy, working-class family in Kirkby, Liverpool. She was the second youngest of 13 children, seven boys and six girls. ”We didn’t have much, but what we did have we appreciated,” she says. “There was never a dull moment because there was that many of us, either fighting or getting on. More fighting than getting on, probably. But we’ve always been there for each other and always will be.”

She left school at 16 and worked in an ice-cream factory. She was never career-minded, she says, believing motherhood would be her biggest role. At 18, she met Ralph Bulger, who worked in security, and they moved into a bedsit close to their family homes. They had been together two years when she became pregnant. But at full-term, Fergus was told the baby she was carrying had no heartbeat. She gave birth to a stillborn daughter, Kirsty – a loss that devastated her. She remembers saying to herself, “Today is the worst day of your life. It will never get worse than this.” Two years later, she gave birth to James.

There were times, she says, when she wanted to die after James was murdered. Initially, she returned to the family flat with Ralph. “But all I could see was James’s toys, his clothes, his bed. I couldn’t cope, so I ended up moving in with my mum for a while.”

In their grief, she and Ralph grew distant. He retreated into a world of alcohol, drinking two bottles of whisky a day, while she retreated into herself. The birth of Michael, just after the trial, failed to heal the rift. One day Ralph said he was going out, and never returned; the couple divorced in 1995. In a book published in 2013, My James, Ralph said that he had blamed Fergus for letting James go, and now felt ashamed.

For a time, Fergus refused to accept that James was dead. “I thought: after the trial, I’ll get James back,” she says. “Once it was done and dusted, he was going to come walking through the door – they’d made a massive mistake. But once the trial had ended and James didn’t come through that door, I knew it was all over.”

She stopped eating. Her family told her she was killing herself. She didn’t care: she wanted to be with James. At his funeral, all she could think about was curling up in that tiny casket with him.

Throughout their trial, reporting restrictions meant that Thompson and Venables were referred to in the press as Child A and Child B. But at the close of the trial, the judge allowed their names to be released, “because the public interest overrode the interest of the defendants... There was a need for an informed public debate on crimes committed by young children.” It was a controversial decision that was to have huge repercussions.

***

Fergus might seem like a tough cookie – indeed, a couple of her friends tell me she is one – but she still finds it hard to talk about James publicly. Today it’s the presence of Stuart who makes it possible. When they met, he was an electrician and just 20 – seven years younger than her. He is a big, warm man with an unwavering devotion to his wife; the more he talks, the more she relaxes.

“I met you in 96,” he says. “In my young, slim, hairy days.”

“He was my toyboy!” she laughs. It’s a nice surprise; I ask if she can remember the first time she laughed after the trial.

“When I had Michael. For the first time I realised I needed to feed him, change him – that this little person was in my arms again and he was mine. There were mixed emotions. I was so pleased to have Michael, but still grieving for James. I thought, I’ve got to stop thinking about what’s happened and start thinking about what’s happening. I had to become a mum again.” She was determined her anger would not poison her children.

Does she find it easy to laugh these days? She looks at Stuart, and grins. “Oh God, you wanna hear me half of the time.”

“She’s one of the funniest people you’ll ever meet,” he says adoringly. “She’s got a wicked sense of humour. When she’s relaxed and in a nice place, she can have everyone rolling around laughing.”

“We all do that, though, don’t we?” Fergus says.

“Yes,” Stuart says, “but it’s nice to see that side of you – people just see Denise, the mother of James Bulger, fighting another campaign or telling people what she’s been through.”

How does she think the public sees her? “I think people see me as a miserable cow who’s constantly fighting and never smiles.”

Does that bother her? “No, because the fight for James is what they see. They don’t see the other side where I let my hair down. I am human.”

When she’s not fighting for James, what does she do? “Moan!” she laughs “Cleaning. Tidying up.”

“Most people’s houses have bottles of wine,” Stuart says. “Denise has bottles of bleach and Dettol.”

Fergus gives him a look you wouldn’t argue with. “No, we have both.”

“True,” he says. “I polish off the bottles of wine, she just polishes.”

“Well, you can take out your anger on the toilet seat, can’t you?” she says.

At times like this, they sound like a double act. But make no mistake: Fergus is still ferocious in defence of her eldest son. She has little time for people who suggest we should try to understand rather than simply condemn Thompson and Venables, because they were a product of troubled families. Plenty of children have difficult backgrounds, she says: it doesn’t turn them into killers.

Two weeks after their trial, Lord Taylor of Gosforth, the lord chief justice, increased the boys’ tariff by two years, recommending that Thompson and Venables serve a minimum of 10 years. Fergus was relentless in her campaign to have their sentence further increased, while their lawyers fought to have it reduced. With the vociferous support of the Sun, close to 280,000 people signed a petition supporting her bid, including 4,400 letters of support agreeing that Venables and Thompson should remain in detention for life and nearly 6,000 asking for a minimum period of detention of 25 years.

After the Sun handed in its petition to increase the sentence further in 1994, home secretary Michael Howard announced that they would be kept in custody for a minimum of 15 years – meaning they would be at least 25 when they were released and would have served seven years in an adult prison. But the House of Lords overturned this decision in 1997, ruling that it was unlawful for the home secretary to decide on minimum sentences for young offenders. Lord Donaldson described Howard’s intervention as “institutionalised vengeance” by “a politician playing to the gallery”.

Two years later, in 1999, the European court of human rights ruled that the boys did not receive a fair trial, because the adult court was “severely intimidating”. Denise and Ralph Bulger applied to the European court, arguing that victims of crime should be involved in the sentencing of the perpetrators. They lost, and in October 2000 the new chief justice Lord Woolf recommended that the tariff be reduced, back to eight years. In 2001, Thompson and Venables were released on lifelong licence, with new identities.

Stuart says this was the lowest and angriest he had seen Fergus. She locked herself in the bedroom and lashed out. What did she do? She looks at Stuart . “I think I just smashed it up, didn’t I?” They laugh. But they didn’t at the time. She felt she had failed in her final promise to James: that she would keep his killers locked up. “I couldn’t deliver and the guilt swamped me,” she writes in the book. “She just needed that time to thrash it out of her head, her mind, her body,” Stuart says today.

Fergus has always condemned vigilantism, but in 2004 she told the News of the World she had tracked Robert Thompson down, with the intention of screaming at him, “Why did you kill my child?” But, “paralysed with hatred”, she could not confront him. At the time she said, “I will find the other one next. They can change their addresses as much as they like – but they can’t change their faces.” Today, she has no interest in tracking them down. I ask how she would cope now if she found herself in the same room as Thompson or Venables. “It would never happen,” she says. “I would never want them anywhere near me.”

***

Back then, there was so much Fergus was angry about: the length of the sentence, the contempt with which she felt she was treated by the authorities, even the way the newspapers insisted on referring to her son as Jamie, when he had always been James.

It’s hard to know what would have pacified her. Fergus writes that “any decent person who makes a mistake should be given the chance to make amends”, but does not extend this principle to Venables and Thompson, because “they aren’t decent people who live by the same morals we do”. She believes giving them new identities means they have never had to own their crime; their time in custody was not spent taking responsibility, but obliterating it. “When they meet somebody, they’ve got to pretend they’re somebody they’re not. In their minds they were told, ‘You never killed that child, you did this instead.’ They had to make up a life, to erase all they did from their minds.”

In 2010 it was reported that, despite being banned from Merseyside, Venables had visited Liverpool several times, where he had gone clubbing, snorted cocaine and attended Everton football matches. Fergus says she was terrified for her family. “The first thing I said to Stuart is, he could have chatted any of my nieces up. He could have spoken to Michael on a night out.”

Her anxiety for her children reached new levels. Five years ago, Michael, then 19, told the Daily Mail he had never travelled alone on a bus or train. The family home in Kirkby is protected by CCTV cameras and security lights.

Thompson and Venables were given new identities because there was a fear of reprisals. Would it have bothered Fergus if they had been killed in a revenge attack? “I can’t answer that because it never happened and I don’t think along those lines anyway.” She points out that Venables has now been recalled to prison twice, for possession of indecent images of children, without being attacked. “If anything does happen to him, it’s not on my head, because I have never come out and said I want them dead. I don’t because I’m not an evil person. I just wanted them to do a proper sentence in an adult prison.”

In 2005, 18-year-old student Anthony Walker was murdered by two young men in a racist attack in Liverpool. He was found with an ice axe in his head after being ambushed. Soon after, his mother, Gee Walker, a Christian, said that she forgave his killers, “because they don’t know what they were doing”.

I mention the case. Fergus, who is acquainted with it, anticipates the question: “She forgave, yes?” Would it have made her life easier if she had found forgiveness? “Of course it would have been easier for me,” she says brusquely. “But it wasn’t to be.” To forgive them would be to betray James, she says. “I am a forgiving person but I’ll never forgive them. Not even on my deathbed. Ask me that question when I’m dying and I’ll still say: I will never forgive them.” She says it with such conviction, like a pledge.

Fergus is still fighting on a number of fronts – for victims’ families to be alerted by the probation service as soon as somebody on licence is recalled to prison, and for an inquiry into why the experts insisted Venables was rehabilitated (she is convinced that those who worked with him when he served his initial sentence knew he had a sexual interest in children). Fergus writes that her friends worry her refusal to stop campaigning is damaging her. Does she think there is any truth in that? “It’s not damaging me whatsoever,” she says. “At the end of the day, I’m the only one to speak out for James. I am James’s voice. And if there’s anything I can do to fight for him, I will.”

A year after James was killed, five-year-old Silje Redergard was beaten to death by two six-year-old boys in Trondheim, Norway. The crimes were compared, although there were notable differences (Silje was older than James; the boys were younger than Thompson and Venables; all three were friends and had been playing together, so it appeared to be a game that had gone wrong). The biggest difference was in the way Norway treated the killers; Trondheim felt a collective responsibility for the murder, and that it was their job to rehabilitate the boys into society rather than to demonise them.

I mention this case to Fergus, too. Again she is familiar with it, and does not find it a feasible alternative. “I think that would be a dangerous thing to do. In their best interests, they should have done prison.”

Stuart points out further differences. “There was an element of this case being accidental,” Stuart says. “With James’s case they caught animals and killed them. They tried to take a girl a few weeks before, they tried to take a boy on the morning of that day. They wanted to do something. It wasn’t accidental.”

After James died, Fergus had nightmares about him. She says writing the book has been cathartic; increasingly, he appears in her dreams as the cheeky, happy boy he was. “It has brought back a lot of nice memories.” For 10 months she allowed everything to pour out to her ghostwriter, Carly Cook, and it served as a form of therapy. “There were more and more things I was remembering about him. Things that I’d put to the back of my mind. I was going home with a smile on my face, and that’s what I needed.”

In 2013, Venables was recalled to prison for possession of indecent images of children – and given another identity. A couple of weeks before we meet, it was reported that he was back in prison, for the same offence. Although Thompson has not reoffended since his release on licence 17 years ago, Fergus maintains that neither was ready for their freedom. “I said years ago, because they weren’t properly punished, that they would go on to reoffend. But I didn’t feel I was listened to. The government thought they knew better.”

Is there a grim satisfaction to be had from the fact that Venables has reoffended? She shakes her head. “I just hope and pray he didn’t hurt anyone else – but I just knew it was going to happen.” What would justice look like now, when it comes to Venables? “He’s not only committed the one crime now, he’s committed three. So he should be locked up for a long time. I’m not saying for ever, just a long time.” Even here, I sense a slight softening. For many years, Fergus called for a full-life sentence for both.

Does she still feel the same intense anger? “I haven’t got the energy for it any more.” Maybe, she says, writing the book has helped get it out of her system. “ I don’t give them the time of day any more. I don’t think about them, I don’t even like saying their names.”

Is Stuart glad she’s less angry? “God, yes,” he says.

Fergus prefers to focus on the positives: her three surviving sons, Stuart, her charity work. The boys are young men. Michael, 24, and Tom, 19, are landscape gardeners; Leon, 18, hopes to be a computer game designer. She says she has finally given them their wings, and has recently noticed a role reversal. “When they used to go out, we’d always be texting and calling, saying where are you, what are you doing, when are you going to be home? Now it’s them calling us to find out where we are and what time we’ll be home.”

Fergus spends much of her time running the James Bulger Memorial Trust, which she set up in 2008. They offer free holidays and respite care in their caravan near Blackpool, to disadvantaged families, victims of crime and those who have suffered bereavements. “We have helped 200 families,” Stuart says. “Because it’s a static caravan we can’t have wheelchairs in there, so we’re now fundraising to buy a lodge where we can have more families.”

I ask Fergus if she takes anything positive from James’s death. “Yes, this. We’re helping other kids in his name.”

Stuart looks at Fergus. “People always remember James as that murdered little toddler from Liverpool, but what you’re doing now is turning a negative into a big positive. James’s legacy is that we’re now helping other kids. That’s given you, not closure but a bit of…”

Fergus smiles, and finishes his sentence for him. “Peace. Yes.”

• I Let Him Go is published by Blink on 25 January. To order a copy for £14.44, go to bookshop.theguardian.com or call 0330 333 6846. A portion of the proceeds will be donated to the James Bulger Memorial Trust, supporting families of victims of crime.
  

Commenting on this piece? If you would like your comment to be considered for inclusion on Weekend magazine’s letters page in print, please email weekend@theguardian.com, including your name and address (not for publication).