Naturally, I had hoped to find Shirley Conran, whose best-selling bonkbuster Lace celebrates its 30th birthday this year, holed up in Monaco with a biltong tan, a toyboy lover and a drinks trolley the size of a small yacht. But no. Apparently, I can put the kaftan and the Jackie O shades away for now. According to her publicist, Conran lives here, in deepest Putney, south London, in a 1930s mansion block in a private cul-de-sac so eerily quiet that it can only be a matter of seconds before a concierge appears from behind a privet hedge and wrestles me to the ground. In the dripping rain, I reach for her bell. Can her flat – I picture it replete with leopardskin throws – really be inside this Poirotesque retirement home?

Safely inside, though, my disappointment evaporates in as long as it takes Conran to make a pot of coffee. For one thing, her apartment is photo-shoot pristine: white walls, white sofas, white drapes, all contributing to a decorative theme we might describe as Fifth Avenue meets Barbados. For another, there is Conran herself. She will turn 80 later this year, but you'd never know it. It's not only her peachy face ("I had my chin and my eyelids done when I was in my 50s," she announces, even before I've turned on my tape recorder), it's her way of thinking, too. Five minutes into our conversation and she's describing the favourite sexual fantasy of British women and why it isn't for her (we'll come back to this, fear not). Ten minutes in and she's recommending a method for finding out whether one's husband is being unfaithful. By the time we've been talking for 20 minutes, we will have touched on masturbation, transvestite husbands and, more prosaically, what she regards as the last taboo for women: the subject of money and how best to get their hands on it.

But first, we must talk about Lace. To mark its birthday, the novel is being republished by Canongate with a new afterword by the author and a wild new cover (cross an old Duran Duran video with an 1980s billboard for Rimmel cosmetics and you're about halfway there). "Oh, it's a fabulous cover, don't you think?" says Conran, with a broad smile. Is thrilled it's being republished? "Yes. Canongate have been wonderful. They've allowed me to write the word 'masturbation' back into the copy. When Lace was first published, Michael Korda [the publishing giant who was Conran's editor] said the world wasn't ready for it. We had to use a polite euphemism. But now we have the verb."

She sips her coffee contentedly. "What I really feel is that, unfortunately, Lace is still relevant." Things haven't moved on for women in quite the manner she hoped they would. "Girls are still expected to be perfect. Boys see girls Essex-ed up to the eyebrows and that's what they expect: no pubic hair and I dread to think what else."

Does this mean that she considers Lace a feminist novel? Certainly, and having reread it myself – I first read it at school, where it travelled like contraband from schoolbag to schoolbag – I'm inclined to agree. I'd forgotten how fiery her heroines are, the triumphant way they throw off – even if it sometimes takes them a while – bad relationships. Its women characters love men and designer clothes (they wear darling little suits by Gucci or flouncy dresses by Zandra Rhodes). But most of all, they love work. Work is a balm. Work is freedom. Work can take a girl anywhere.

The book is about four friends. The story begins at a finishing school in Switzerland, where they first meet shortly after the war. It ends in 1978, in Manhattan's most exclusive hotel, where a young woman shouts at them: "Which one of you bitches is my mother?" In her afterword, Conran writes that all her characters existed in "real life", that only her plot was "complete invention". Maxine, who turns her husband's champagne estate into a multimillion-pound hotel operation, was based on a girl at Conran's finishing school and her career inspired by the transformation of Woburn Abbey (the Dowager Duchess of Bedford is an old friend).

Pagan, who recovers from alcoholism to become a leading fundraiser for cancer research, was inspired by another schoolfriend, while Pagan's lover, Prince Abdullah of Sydonia, is basically King Hussein of Jordan (as a young woman, Conran spent two years chaperoning her friend while she and the king, then a Sandhurst cadet, dated). Judy, who becomes one of America's most successful PRs, was modelled on Betsy Nolan, the publicist who first encouraged Conran to write a novel. As for Kate, a war correspondent who goes on to set up a successful women's magazine, Verve!, she is Shirley in heavy disguise.

But the book began its life as non-fiction. By 1983, Conran was already the author of Superwoman, the bestselling housework manual in which she told women life is too short to stuff a mushroom; Forever Superwoman, which was about having young children and remaining sane; and Futures, a guide to the menopause. Her plan was that her next book would be a guide to sex for schoolgirls. "The ignorance was so abysmal. I spent 18 months researching it. But then I got so bored I thought I might as well have a go at writing a novel. So Lace is really intensely researched sexual information dressed up as a novel."

Ah, yes. The sex. The naughty bits in Lace are pretty, well, naughty and certainly a good deal more satisfactory than anything in EL James's Fifty Shades of Grey. The bit where Judy ties her married lover to the bed and covers him with olive oil is particularly, er, well drawn. So was it…? My voice trails off. Oh dear. How to put this? Conran gives me a look. "Are you asking me if it was sexually exciting to write?" Er, yes. I suppose I am. "No, it was not. I did not find it erotic. Maybe some people sit at the typewriter having masturbatory fantasies. But not me. I'm too intent on my spelling."

Has she read Fifty Shades of Grey? "Yes." And? "I don't want to slag another writer off, but basically, it took until page 100 for her [the heroine, Ana] to be whacked with a hairbrush and until page 400 for her to be hit with a belt." She laughs. "I saw my father administer these punishments to my brothers when they'd done various naughty things... I'm sure that never influenced their sexuality. Look, we have to salute the achievement that it's selling millions, but I can't think why. The writing is so jerky." She sighs. "I'm going to sound awful, but a favourite erotic fantasy of British women is sex with a schoolmaster in a corn field. I mean… all those bits, sticking in your bum! I hope the schoolmaster goes underneath. You see, I'm a very practical person, being a Virgo. These are the kind of thoughts I find intruding on me. Not erotic at all."

Was she surprised Lace was a hit? (She sold 3 million copies and it was made into a mini-series starring Phoebe Cates.) "I was relieved." At the age of 40, she had gone into hospital to be treated for pneumonia, only to emerge a month later with ME, a condition that meant, in the end, that she had give up her beloved job as the editor of the Daily Mail's women's pages. "Until I had ME, I'd never bothered about money. My father was a successful dry cleaner, with factories and shops. Consequently, I didn't bother about it until I didn't have any. So it was a relief when I discovered that I could write fiction. Then, it was just fun. I got to go to New York on Concorde. My publisher paid for me to go first class, but I always paid the extra so I could go on Concorde."

There followed five further novels, most famously Savages, in which five women travel to a tropical island, only to see their business executive husbands murdered during a military coup. The women escape into the jungle, where together – roar! – they learn the art of survival. The final novel (The Revenge of Mimi Quinn) was published in 1998, at which point Conran decided to return to the real world. "In novels, you control everything. Everybody does what you want. So I retired. For about three weeks. I went to stay with some friends in Palm Beach. They'd been very dynamic before they retired, but in Palm Beach… well, instead of making one big shopping list a week, every time they ran out of anything, they would trundle off in the car to get it. NOT FOR ME!" she shouts.

So she went back to work. First, she set up something called Mothers in Management. Then she established a charity, the Work-Life Balance Trust (it's now defunct, flexi-time having become, she says, a widely established concept). Today, she's still hard at it. An ebook she has written about women and money will be available later this year. Is she still rich? Lace, for which she was reputed to have received a record advance, bought her, among other things, a glorious 10th-century chateau near Cannes; in 1994, she was said to be the 84th richest woman in Britain. "I'm not anxious about where the next penny is coming from, no." She picks up a ginger biscuit and, with great precision, takes a tiny bite.

Conran was kicked out of home at the age of 19 by her father, who was a violent alcoholic. "He was in one of his states," she says. "My mother could do nothing." So off she went to London. "Unfortunately, I wasn't qualified to do anything. I collapsed with malnutrition after two months and had to be taken back to Portsmouth. I was put to bed and my father wasn't allowed to see me. Ah, the relief! It took me a month to get over that, then I went back to London. My mother slipped me £3 a week, but it didn't pay for much. I lived on leeks." A well-timed pause. "I still like leeks… anyway, I was by now so thin, I could work as a model and when I'd earned enough money doing that, I went to art school in Chelsea."

It was in Chelsea that she met Terence Conran, her first husband, and the father of her sons, Jasper, the fashion designer, and Sebastian, a product designer. Did she see him as a kind of escape route? "No. Actually, I was in love with someone else. I thought Terence was gay. Quite a lot of people used to think Terence was gay. But in fact, he was not at all gay." The other man wanted her to get pregnant; only then, once she had proved she was able to conceive, would he marry her. So she left him.

"Terence had opened a coffee bar at the end of the street my college was in. I used to go along and waitress for a meal and he used to give me a lift back to his flat at two in the morning, and after a bit, guess what?" Did she think he would make a good husband? "No! Nobody did. But there you are. I thought he was the right choice. He was the right person to be the father of my children, but people hadn't really sorted out what marriage was then. Fidelity, and did it count, and all that. This was the late 1950s. We were still very much corseted."

They were married in 1955 and divorced in 1962. She worked for her husband, who would open his first Habitat store in 1964, as a designer, but he rarely confided in her when it came to business and there were many affairs. Suspecting he was having one with his secretary, Shirley gave her a box of Roger & Gallet carnation soap for Christmas in the belief that she would duly smell the scent on her husband. Did this trick work? "Oh, absolutely. They've just put it back on the market, in fact. So.... TOP TIP!" Finally, she issued an ultimatum. The philandering had to stop. When it didn't, she left. "I was 30. My mother said, 'Well, perhaps it's as well your father's dead because otherwise you couldn't have got divorced.' I gave her an astounded look, but that's how it was then."

How did she get up the courage to leave? "I went to St Paul's [the independent school in west London]. That was very important. The mistresses were all unmarried – their fiancees had all died on the Somme – and they were quietly subversive. Paulinas are known for having their own views. It would be very difficult for a Paulina not to expect equality in marriage. So when you get married, and find it's not so, it's a shock."

This is why, she says, all her books are "for women, to improve their situation one way or another". She had to go to court several times to secure maintenance from her ex-husband and at first was only able to survive with the help of her mother, who looked after her sons in Portsmouth while Shirley worked as a journalist in London (she edited the women's pages of the Observer). She married twice more, but both relationships ended in divorce. "After that, I was sick of being in the vanguard: first to get married, first to get divorced, first to get married again. It wasn't a conscious decision, but I decided to remain single."

She likes being able to please herself. "Quite recently, I was heavily courted by someone extremely eligible," she says. Oh, really. Was he younger than her? "Of course! Nobody's older than me. He was 70. A gorgeous 70. We had an affair 30 years ago, which he remembered in somewhat embarrassing detail. Anyway, immediately this started, I began thinking: maybe I should go on a diet and so on. Little self-improvements. The second a man comes into your life, you aspire to be good enough for him. Finally, I thought: oh, f*** it. I'm happy as I am." So they split up? "Yes. He didn't like me enough. I think he thought I was bit vulgar. I considered this and I thought: well, maybe I am. I choose to be. But I was being a little woman, you see, and that was interesting to me. The Paulina in me stood aside and said, 'You're doing exactly what you did when you were 17.'" She sighs. "I think we're all 17 in our heads, forever." How did she feel when it ended? "I felt relief. I suddenly realised I might be answerable to someone else. He might want to watch different telly. He might think some of my projects, like funding little art scholarships, extravagant."

When she was a single parent, it was she and her sons against the world. Are they still close? "I haven't seen Jasper for 10 years," she says immediately, straightforward as ever. I gasp at this. Have they fallen out? "No. But he won't tell me what the problem is. One day, I read a quote in a newspaper from Jasper saying, 'I used to have a close relationship with my mother, but now we have a distant one.' I thought: I'm going to want an apology from the journalist for this. I rang Jasper's office, but I couldn't get through and eventually I established this was the case [ie the quote was accurate]. We have seen each other in a crowd a couple of times and he said on one occasion that he knew I wanted to know what it was about, and that he would tell me, but he hasn't done so." This must be devastating. "Yes. I spent two years in a depression about it. But now I deal with it by thinking: I'd rather he was successful and happy and didn't see me than he was dead or something. I was lucky to have a devoted son for 40 years."

What about Terence? She often talks about him – part of me thinks she never quite got over him – but does she still see him? "With Terence, you're in one year and out the next. I got invited to his 50th and his 60th, but not to his 70th or his 80th. I can't help but wonder if it depends on how famous I am at the time." In her afterword to Lace, she writes that he had a lawyer read the book twice to check she said nothing libellous about him. "Yes!" And he found nothing? "No." But there is a character, isn't there, who's a little bit Terence-like, a designer who guts his terraced house, the better to turn it into a modern "living environment"? She nods. Only this character turns out to be a transvestite. "Yes." How mischievous. "I'm always mischievous!" She looks like the cat that got the cream.

An assistant appears: her morning secretary (there is an afternoon one, too, and a part-time cook, who is her "one indulgence"). "Just to remind you about your telephone call at two," the young woman says, efficiently. Right, I'd better go. Shirley, though, hasn't done with me yet. At the door, she hands me a parcel, wrapped in tissue and ribbons. "These may not be your taste," she says. "So you'd better open it now."

Oh, crikey. Inside, is a pair of shorts (size: LARGE) made of white lace. They are from Zara, which means that, though she took the precaution of buying them on the roomy side, they are teeny – the kind of beauties that would look excellent on Lara Stone or Sienna Miller but not, alas, on me. Thank you, I say. I kiss her, the better to hide my expression. But she isn't done yet. "You must wear them in bed," she says, with reforming zeal, as I grab my umbrella. "They're the new nightwear!"