Marie Helvin joins Tracey MacLeod at the new Leith's in Soho
Only after I'd arranged to meet Marie Helvin for lunch did it occur to me that top models tend to have the same relationship with food as Superman has with Kryptonite. Their equivalent of the three-martini lunch is the three-calorie lunch - cigarettes, mineral water and black coffee. And when I then discovered that, since quitting full-time modelling, Marie has published a book about fasting, I was resigned to the prospect of eating for two.

But as soon as she swung her way into the sunny dining room of Leith's Soho, it was plain that Marie isn't the kind of model who believes in self-deprivation. Enviably slender, but still fantastically curvy, she was dressed in skin-tight black from head to toe, and looked so impossibly feline that I expected our waiter to bring her a saucer of cream, instead of the basket of characterful bread varieties which arrived. Explaining that she fasts only on Sundays, Marie plunged into the breadbasket like a good 'un, while chugging down the first of several glasses of Chablis. "The rest of the time I lerve to eat," she purred in her soft American drawl.

Leith's Soho is the latest in a series of high-profile restaurant launches around Golden Square, the throbbing new heart of Soho medialand. The kind of place where you do, rather than have, lunch, it's a sharp-suited younger brother of the original Leith's in Notting Hill, which was upmarket and fantastically expensive long before it became obligatory in that ultra-fashionable quartier.

The new Soho branch offers a streamlined version of the adventurous contemporary Leith's style, and regulars of the original will recognise its approach in the vegetarian-friendly menu and lovingly chosen wine list. The menu is the creation of the Leith's Michelin-starred chef Alex Floyd, who is now cooking at the Soho branch. His unstuffy style is exemplified by this cheery message on the menu - "If you'd like something really simple, please ask. If we have the ingredients in our kitchens, we'll be only too pleased to prepare it for you."

In terms of design, Leith's Soho is the very model of a modern London restaurant, all blond wood and burnished earth tones. Undulating coppery walls, suede-seated birchwood chairs and back-lit smoked-glass panels create an atmosphere of expensive Californian sleekness. The room is small, with seating for about 50 diners, and the shape is not particularly convivial - for most of its length it narrows to little more than a corridor of single tables. But diners at the desirable window tables are well placed to watch Japanese film crews struggling up the road to nearby Carnaby Street.

Marie's presence, though, ensured that most of the diners weren't bothering to look out of the windows. She seemed oblivious of the collective neck- craning and nudging she provoked as she surveyed the menu. "Wow, this looks faaabulous!" she enthused, with all the practised charm of someone not unused to being taken out for expensive meals. Tempting though it was to test out the chef's offer to make us something really simple, the dishes on offer sounded so good we decided against going off piste. Marie began with shiitake mushroom and cos lettuce spring rolls - three neat filo parcels prepared and presented with Oriental precision. As someone who has spent most of her life in a world where appearances are all, she was inclined to judge her food as much by how it looked as how it tasted, and her starter scored highly on both counts.

I chose to start with prawn cocktail, already the restaurant's most requested item and, like my guest, a dish hugely popular in the Seventies still winning new fans today. Temptingly presented in a large martini glass, its rim frosted, margarita-style, with paprika, it was as winningly sweet and satisfying as only prawn cocktail can be. "I wonder why they call that sauce Marie Rose?" Marie mused. "In America, it's just called ketchup'n'mayo."

When I first met Marie in the mid-Eighties, I was working as a researcher on a chat show, and she was booked as a guest, to promote her book, Catwalk. The producers were wise to entrust her to my care, because her presence in the green room robbed the programme's male personnel of the power of speech. She had the same effect on my then boyfriend a few years later, when we flew up to Scotland together for a friend's wedding. While he trotted after Marie at the airport, offering to help her carry her tiny vanity case, I brought up the rear, struggling with three heavy bags and a guitar case.

We've seen each other only intermittently since, so while we ate, we updated each other on our romantic CVs. On her side, this involved such a stellar cast list that I had to turn off my tape recorder, for fear I might one day be tempted to sell the tape to News International if times were ever to get really hard.

My main course of roast scallops with lemon-spiked couscous was a paragon of contemporary fusion food: the Middle Eastern notes in the couscous harmonising well with the smoky complexity of an Indian-influenced fennel and turmeric sauce. Marie had chosen salmon and crab cakes, perhaps imagining Thai-style dainties. What she got instead was decidedly English - one big, solid fishcake, served on a spinach bed and surrounded by a bland and over-buttery tomato sauce. "It's a bit too butch for me," she said, though she still ate a healthy amount. She was more enthusiastic about our shared rocket and asparagus salad, in a powerful Roquefort dressing that gave the rocket an Apollo-sized lift.

The pudding menu is short and includes a selection of traditional and New British Cheeses (first we had Young British Art, now we have New British Cheese). I thought Marie had reverted to model type when she revealed that she never normally eats desserts in restaurants, but she explained that this was because she eats so many sweets at home - "I had a Kit-Kat this morning. They've brought out an orange-flavoured limited edition, so I bought a box of 'em."

We opted to share a banana tarte tatin, which turned out to be distinctly pornographic in appearance. Three spears of banana, caramelised at one end and grey at the other, projected from a pastry sheath, topped with a ball of ice-cream. "This reminds me a little of my breakfast," Marie said, with a secret smile that made it plain she wasn't talking about the Kit-Kat.

As I paid our bill, which came to around pounds 80, I noticed the chairman of Leith's Soho, Sir Christopher Bland, holding court at a nearby table. Sir Christopher is also the chairman of the BBC, and for a moment I toyed with the idea of testing the "if you'd like something really simple, please ask" principle, by asking for my own series on Radio 4.

Marie's own ambitions include a long-cherished plan to open a restaurant. "It would be more of a club than a restaurant," she explained. "People would be able to lie around on sofas, and we'd serve them delicious finger food, like caviar and sushi - it would be really sexy." We agreed that there wasn't anything particularly sexy about Leith's Soho, but that for a business lunch, it was a very acceptable new option.

As we hit the street, a passing taxi-driver spotted Marie and immediately shrieked to a halt, in the hope that she might wish to be driven somewhere. She jumped in, on her way to meet Marco Pierre White at the Mirabelle for an afternoon of continued hedonism. "Hey, Tracey!" she called out as we parted. "I'll send you my book on fasting! You'll love it!"

Leith's Soho, 41 Beak Street, London W1 (0171-287 2057). Mon-Sat 12-2.30pm, 6pm-11.15pm. All credit cards, except Diners. Disabled access, but not to toilets. Private dining room for 15-20.