Model behaviour: Marie Helvin

"I feel like a blimp," says Marie Helvin, sucking in her cheeks so far, it looks like she'll swallow her nose. "Here goes nothing." She rocks lightly on the balls of her feet, a high-diver testing the board, and steps before the camera. You can almost hear the whirr of an internal motor as she goes through her moves, striped in shadow, like a film-noir Coppelia. Arm across body, hip thrust to contraposto, lips pushed, in three controlled stages, to fullest pout. An eyebrow is raised and lowered with hydraulic precision, limbs slide and lock just long enough for the photographer to breathe his appreciation. And all the time, Helvin stares out, through the closed curtains of the hotel suite, at a fallen world. She looks transfigured, faintly tragic, like someone who has briefly glimpsed a better way. Or maybe she's thinking about a dead puppy.

"Dry cleaning, actually," she says, waving her post-shoot glass of champagne. "I was thinking about my dry-cleaning bills, the fact that my bathroom floor needs re-tiling and what I'm going to have for lunch. Interestingly enough, I have never once, in my whole career been photographed while thinking about sex. That's what photographers like to think you're thinking about. You know what they want, so you give them what they want. You make the faces they think are sexy. But to me, it's Look No. 4, or Look No. 6. It's a performance."

It's almost 30 years since Marie Helvin first shot "the look" to camera. In that time modelling has gone from something nice girls did between secretarial school and marriage to a multi-billion-pound industry churning out glamorous monsters with a shelf-life shorter than a (low-calorie) sandwich. In Models Close-Up, the documentary series shot by her ex-husband, the photographer David Bailey, (currently showing on Channel 4) Helvin's brief, breezy appearances are a welcome relief from the spectacle of supermodels

gazing at their neatly pierced navels and banging on about the mental and physical discipline required to pull on a pair of trousers.

Helvin doesn't try that hard. When we meet for a picnic in the Pimlico garden square where she has an apartment, I bring along a fat person's idea of a supermodel's lunch, all grilled veg and designer water. Marie lobs in the Kit-Kats. She buys them by the catering pack. And she would rather talk about the new memoir of JD Salinger than The Trouble with Thongs. Her flat, heavy with the scent of roses, is crammed with teetering columns of books and she is constantly revising her recommended reading list. Biography is a particular passion and it is probable that when the "lives" of the late 20th century are written up, Marie Helvin will feature in a good many indexes; Marie knows everyone and everyone, it seems, from hotel doormen to cabinet ministers, loves Marie. Until posterity kicks in, however, she is a stalwart of the gossip columns. Just recently she arrived at a party with her good friend Salman Rushdie and left with Neal Pearson. Her name has been linked with Imran Khan. And in a weird, fatal loop, she has had affairs with both Mark Shand, brother of Camilla Parker Bowles, and Dodi Fayed. The old KGB would have paid a fortune for somebody with Helvin's contacts, but she is not inclined to dish.

"I'm not interested in writing some gossipy book about fashion - who did what drug where - and I'm certainly not interested in doing `the real story' of Life with Bailey. The man has kids, for God's sake!"

"Bailey." Helvin names her demon with exasperated affection. The couple met when the 19-year-old Helvin turned up at Bailey's pad (the "snappers" of the Seventies all had "pads') for a photographic session. The shoot took place in Bailey's bedroom, on his bed. "There was nothing strange in that. It's how things were," says Helvin, without nostalgia. "The strange thing was that we got married. Nobody got married in 1975 for God's sake! But I guess I needed a British passport."

Born in Tokyo, where her American GI father had married a local interpreter from the Officer's Club, Helvin moved to Hawaii at the age of four. The Helvins were not an orthodox family. Father was a health-nut radical who urged his children to cut school and made them send Christmas cards to Joan Baez in gaol. Mum did her best to fit in (confronted with her first tea-bag at a wives' coffee morning she politely ate it) but developed a caffeine addiction. Advised by her doctor, bizarrely, to swap coffee for marijuana, she shared her new tonic with her teenage children. Marie, the eldest of four, learned the traditional philosophy of Hula, grew up without a shoe to her foot and was sent home from school for wearing her skirts too short. At 15, on a trip to Japan with her mother, she was "spotted" and signed up as the face of Kanebo cosmetics. Her sights, however, had long been set on London.

"I was addicted to The Monkees TV programme. Not so much because of the music but because of the commercials in between. The programme was sponsored by Yardley and in the commercial breaks there would be these English girls, on roller skates, wearing hot pants, and I just thought. "God! How neat!" They were like the most beautiful alien creatures I had ever seen and I wanted to be like that."

London was equally charmed by Helvin's exotic look (a Eurasian face was a daring departure on a 1970s magazine cover) and soon she and Jerry Hall were the crowned queens of the catwalk. "They called us the terrible twins and we felt like we owned the world. We could get away with murder, demand any amount of money, demand this, demand that. Now I blush to think how outrageously rude we were, but I hope I didn't really hurt anyone's feelings. You have to remember that we were both with very, very strong men - I was with Bailey, Jerry was with Mick (Jagger) and it was only when the two of us were out together that we could actually seize some power of our own. The minute the men went out the door, it was, like, `Yes! What are we going to do now?'"

These days, Helvin is the very soul of genial self-possession. She squeals with delight over the clothes chosen by the stylist for today's shoot and tries them on like a delighted teen at a pyjama party, but at the last minute she pulls her own slip dress from a Prada holdall the size of a small garden shed and wears that. She stands like a lamb while light levels are fiddled with, but she knows when it's right when she feels the heat on the tip of her nose. It is hard to picture her submitting to Bailey's Svengali-ish notions, but the evidence is there in black and white. Trouble and Strife, Bailey's collection of photographs of his then wife, for the most part naked, bound and gagged, caused a furore in the art world which has never quite died down. "Boy, what a mistake that was," says Helvin. "It'll haunt me to the day I die. But I was in love with my husband who wanted to photograph me in the nude and I was stupid and naive enough to agree to it. It never occurred to me that they would be published. That's how stupid I was. And then he said (whining weasel voice) `I'm thinking of putting some of these pictures we've been doing at weekends into a book,' and I said, `Oh, yes, sure.' By the time I realised that it was actually happening there was nothing to do but walk away from it. And to the day I die there will always be some asshole who will come up to me in a supermarket and ask me to sign one of those fucking pussy pictures. There was one in particular, where I was completely naked, wrapped up in newspapers, spreadeagled, thinking, `Please God, why am I doing this? Why can't I just say No?'" Just once, Helvin rebelled. "Bailey and his sponsor, Olympus, were after the ultimate photograph. They wanted me to go into hospital and swallow some photo-optic cable so they could photograph my womb. I guess it was a kind of possession. In all the time that I was with Bailey, he was never, ever without a camera. He slept with it on the bedside table. It wasn't so much an instrument to him, as an extra sense, like a third eye or an extra cock. And," says Helvin smoothly, almost sweetly, "he just couldn't put it away."

She can laugh, now, at the irony of the constitutionally unfaithful Bailey divorcing her on grounds of her adultery. Helvin has never made great claims for monogamy. "I think I'm living in the wrong century," she says. "I would have made a great courtesan. Not a mistress - I could never be kept - but a courtesan with my own rules."

Helvin's candour on these matters could wrong-foot the sternest critic. She declines to talk about her recent affair with the actor Neal Pearson, but this, you sense, is more to do with honour than self-preservation. There is no wistfulness, just a kind of shoulders- back honesty when she says, "I'm good at many, many things, but the one thing I've never been good at is relationships. I've had moments, and some major lovers, but I never really had a great relationship. If you believe, as I do believe, in reincarnation, they say that every time you come back, it's because you have to finish something. I've a funny feeling that my thing in this life is to have one relationship after another. Perhaps, in a former life, I was some real horrible bitch who was just so horrible to men that now I am being paid back for it."

The belief in reincarnation, the spiritual retreats, the doctrine of fastings and colonic irrigation (Helvin is the author of Bodypure, a best- selling beauty de-tox plan) are all unfeigned but Helvin has the appealing knack of being entirely sincere without taking herself entirely seriously. "Look," she says. "What happens to a model when she's not a model anymore? Either she goes completely anal or she goes dippy. I guess I'm taking the dippy route." At 46, Helvin betrays none of the edginess common to beauties d'un certain age. Partly, one suspects, because she still has the body of a fortunate 20-year-old, and partly because she genuinely sees her looks as a gift, or a professional resource rather than as an attribute.

"I'm a freak of nature," she says cheerily. "I know I don't look my age but it's not something I aspire to. Sure, I want to look good - it's my job to look good - but I don't necessarily want to look younger. Everybody knows how old I am. If I was going to start lying about my age I shoulda done that 20 years ago. Of course I look after myself - I diet now, which I never used to and I go to the gym - but not more than anyone else. My mom, who is 72 and looks about 50, warned me that women with our Mongolian blood stay beautiful-looking longer than most, and then, overnight, they look 2,000 years old." Helvin looks down at her frail, ringed hands, the hands of a Ming empress, and gives the smallest shudder. But she's not about to hang up her Blahniks just yet.

"I'm still waiting for that one big job, that one big contract that will make my fortune. I know that when I say that it's like, `What? You're 46 and you still think that's gonna happen?' and yes, I do, I really do."

Meanwhile she is broadening her options in the media. She is co-writing a comedy series based on the fashion industry with her friend the novelist Kathy Lette, and is in negotiation to present a chatshow on network TV. "And maybe," she says, "if Playboy says to me "Go on, Marie, get your kit off', maybe I should quit angsting and do it. My attitude is: `Make me an offer and let's see if I can refuse it.' Because at the end of the day, I'm on my own. I live alone. Nobody pays my bills and hell, I've got a ranch to buy."

She already has the place earmarked, a tract of land on a small island near Hawaii. "Someone asked me a couple of nights ago, "But Marie, what are you going to do there? How do you see yourself on this island?" And this really shocked me, because you know what I said, the first thing that came out of my mouth?

"I said `Fat'," says Marie Helvin, rolling the word round her palate like a whole new taste sensation: "Fat and happy"

John Stoddart's photographs of Marie Helvin were shot at One Aldwych, London WC2 (0171-300 1000). Stylist: Julia Clancey at Carol Hayes. Hair: Benjamin Ahrens at Hari's.