It is easy to feel like the worst kind of tactless idiot when interviewing someone like Sara Payne, mother of eight-year-old Sarah, who was playing with her brothers and sister in a Sussex field when she was abducted and murdered by Roy Whiting on 1 July 2000. What can you say? 'Sorry' is ridiculously inadequate. 'What's it like to be the mother of a murdered child?' is crass.

Part of the problem is that now Sara Payne is so recognisable, we, the British public, think we know how she feels. We can't, of course. How could we? But we feel we can, just as we felt empathy for her when she first stood, ashen-faced but controlled, making her televised appeal to whoever had Sarah to let her go. 'Don't be scared or frightened,' she said. 'We are not concerned with you. Just drop her off and let her walk away from you.'

I remember thinking, 'That's good, that's clever.' Not raging or lamenting, or calling for the death penalty, but soothing and calming, trying to establish some kind of one-to-one with the abductor, get across the idea that there was still a way out, a chance to step back from the darkness.

For Sara, it was also a means of communicating with her daughter. 'I always imagined Sarah was watching,' she says, pushing back her long dark hair. 'I needed her to stay calm and believe we were coming to get her.'

She brushes aside the idea that her self-control was extraordinary. 'It's just being a mother, isn't it? I totally understand why people lose it, but the only way I could handle it was to imagine that Sarah was watching me. I wanted her to know we were in control, we were doing everything possible, and we were missing her.'

Now Sara has published Sara Payne: A Mother's Story, her account of the last four years, as told to a journalist who was there from the beginning, when Sarah was taken. Settling down to talk to me at her publishers, accompanied by her six-month-old baby, Ellie, and a family friend, Sara is at pains to point out that the book is just from her perspective, and she is not speaking for her husband, Michael, Sarah's older brothers, Lee and Luke, or younger sister, Charlotte. 'They all have their own perspective and I'm sure their story would be almost a different story.'

She has often been approached to produce a book but she wasn't sure until she became pregnant with Ellie. 'It just seemed like a good way to end this chapter, say how it really was.'

Some may feel disquiet at the thought of a mother writing a book about her child's murder. Does Sara anticipate criticism? 'I suppose some won't like it or won't agree with me writing it. But if you want to read it, read it. If you want to know the truth of how I think, there it is.'

Was it a painful process? 'The opposite, actually. I found it very helpful. It put back some timings and memories and also some stuff that I didn't want to remember but needed to.'

Her manner is reasonable, down to earth. Sara, 35, is so calm and together when she talks, so matter of fact as she carries Ellie off for a nappy change, it is easy to forget why she is here, and what she is talking about. Then you catch a look on her face - grey, drawn, blurred - and you remember that not only has she lived through something that would finish most of us off, she has lain on the very floor of hell while she was doing it.

The opening chapter of the book serves as a 'Before' snapshot of a contented, uncomplicated family just before their world was ripped apart. The Paynes had children very young, there wasn't much money around, and Michael suffered bouts of manic depression. But they were doing OK, they were happy. Sarah, their 'princess', was a caring child, a 'girly-girl' who loved dresses and fairies, and still insisted on believing in Father Christmas. She also believed in God, debating the matter with her Darwinian-minded elder brother. 'She was a real softie,' says Sara. 'I don't know how she would have coped with the world as she got older, to find out it wasn't as rosy as she thought.'

In the book, Sara remarks that she could never quite picture Sarah as an adult. 'I'm not saying it was a premonition or anything. I just don't think she would have fitted in as an adult. I think she would have always remained an innocent. Shehad that way about her.'

When Sarah disappeared, it took Sara a long time to accept that someone had taken her, and even longer to accept that she might be dead. She barely ate or slept, existing on hope, adrenaline and denial. When Sarah's body was finally discovered, the Paynes were advised not to see her because after 17 days exposed to elements and wildlife, it wasn't 'Sarah' any more. Sara sees that as one of the most unforgivable things Whiting did. She's unable to comprehend why he couldn't have made an anonymous phone call so that the body could have been recovered earlier. 'He not only took her, he took away our goodbye as well. It's just the most inhuman thing.'

After burying their 'princess', the Paynes had to deal with Whiting's trial, which Sara and Michael attended every day because they felt Sarah needed to be 'represented'. Sara also wanted to stare Whiting in the eye. 'I wanted him to know he didn't scare us. Even though he might be scary to little children, he's not scary to adults.'

What did she feel when staring at him - hatred, contempt? Sara shakes her head slowly: 'Just numbness.'

Whiting was sentenced to 50 years, but long before this Sara had immersed herself in the campaign for Sarah's Law, inspired by Megan's Law in America, to set up a register to let parents know if paedophiles were living nearby. This led to the News of the World naming-and-shaming campaign, culminating in public hysteria, vigilantism and, farcically, the targeting of a paediatrician. Sara condemns the vigilante action but stands by the News of the World. She believes their support for Sarah's Law gave her the credibility to walk into meetings with heads of departments, government agencies and powerful politicians such as David Blunkett and Jack Straw. She has never felt patronised or out of her depth. 'You've got to know what you're talking about,' she says. 'You can't be the wailing , mourning mother - you've just got to get on with it.'

Just getting on with it seems to be the key to Sara. She says she is 'very different' now. In the book she comes across as a laidback sort before the tragedy, happy to be a wife, mother, daughter, friend, to work as a barmaid, play with her children. Had I met her then, I probably would have thought her bohemian, non-materialistic, sorted, admirable. Now 'admirable' seems not big enough. (If you ask me, people in her position are 'admirable' if they manage to comb their hair in the morning.)

She has had to become self contained, articulate, determined and perhaps when necessary a public persona where a person should be. (How else could she cope with a life lived through the filter of her child's death?) She really is 'just getting on with it', to the point where she seems bemused by the way people admire her for her composure. 'It's just the way I am,' she insists quietly. 'I'm not someone who mourns publicly. I can only let go on my own.'

By her own account, Sara's impressive strength in public was not being matched at home. She 'lost the plot', alternating between over-protecting her children and neglecting the normal motherly roles. She let the house turn into a tip, often sitting in her dressing gown, chain-smoking, watching daytime television for hours.

Maybe this is the true value of Sara's book. Along with the big issues, it illustrates the sheer mess murder can make of daily life: dirty pots in the sink, stale milk in the fridge. 'It is messy, it is dark,' says Sara. 'You spend your life going to places you don't want to go.'

Sarah's father, Michael, had coped less well in public, standing puffy-eyed, devastated and all but mute beside his wife - but he was the one who tried to hold it together at home. Sara smiles ruefully: 'One of Mike's biggest things was that everyone thought I was so together and strong, and the truth is I'm not. I can't cope with the fact that my child has been murdered. I don't think anyone could.'

Michael had to cope with his wife's near-iconic public status as Sarah's mother, and the sidelining of his own pain. Sara says she sees it all the time with her work for the Victims of Crimes Trust. 'I feel sympathy for any man in the situation. It's not geared to dads. It doesn't take into account their emotions. They have no one to turn to.'

The strain took its toll on the Paynes' relationship, and they split up. 'I think Michael felt neglected, and he was. We neglected ourselves, and each other.' There was too much guilt and booze, too many arguments. Their minds constantly churned with recriminations. 'I'm always questioning myself about why I left Sarah that day,' says Sara. 'If I hadn't, would she still be here? But that's a question you can try and answer for the rest of your life.'

Michael's grief and impotence turned to rage. One day he got a gun, wanting to kill Whiting, and Sara had to get the police to talk to him. 'I wouldn't blame Michael. I want to make that clear. But if you did that to another human being that could make you a very different human being. And I don't know if I could live with someone who could do that.' A failed suicide attempt led Michael to seek treatment, and she says he's doing much better now. Will they get back together? 'I hope so.'

The arrival of Ellie changed the family dynamic for the better. At first Sara was unsure whether she was mentally strong enough to cope with the pregnancy, but the miscarriage of one of the twins she was carrying convinced her. 'It was like a big kick up the bum: She's here, she's staying, sort it out!'

What impact has Ellie had? 'She's given us back tomorrow. Before she came we lived day to day. If you haven't got tomorrow there's not much point in today. You might as well jump off a cliff.'

It is almost time for Sara to begin the journey home to Surrey. She tells me that had Sarah's body not been found, she wouldn't have left Sussex - she would have stayed there for ever, a willing sentinel, stuck in time, always half expecting her daughter to walk through the door. As it is, four years on, she is learning to live with her death and the strange and terrible fame it has brought her. She will look after Ellie at home. 'I'm not good at handing them over. I never was.' She will try to be the fun, fair mum her children knew before. But she still feels guilty when she catches herself having fun - 'stupid things' like playing pool or talking to friends. 'You laugh, and for a second you let go. Then it comes crashing down.'

When there is another high-profile case, such as Soham, she says she's horrified but tries not to be negative - after all, she says, not all abducted children end up murdered. As for Sarah, she doesn't find it upsetting to think about her. 'It just saddens me she's not with us. What makes me angry is thinking about him, so I try not to.' Sara was brought up a spiritualist, which she says helps. 'I know death is not the end for me and Sarah. I will be with her again.'

Before we part, I ask if Sarah's Law was an attempt to wrest some good from bad? She says she wanted to do something positive, but adds: 'It's also selfish. I don't want Sarah to be forgotten. I don't want her to become a statistic, and people look at her face and think, "Who's that?" As it stands, nobody's forgotten her name or her face.'

· Sara Payne: A Mother's Story is published by Hodder on 24 May, price £16.99.