Jane Asher kicks off her scarlet patent stilettos, stretches her black-stockinged legs across the hotel sofa and lights a cheroot. "I can’t tell you what a relief it is to talk about all this at last," she says. "My psychiatrist suggested it. Same idea as Jamie Lee Curtis being photographed in her underwear. Complete revelation. So I’d like to start with the whole Paul McCartney thing - just to get it out the way. Then my marriage. Then how much I hate cakes. Is that okay with you?" I smile a dreamy, appreciative smile, and switch on the tape recorder.

It wasn’t like this, of course. Not the stilettos, not the cheroot and not a chance of the true confessions. But it was Jane Asher whom I had arranged to meet. She of the Titian hair, milky skin and Bambi slenderness. She of the impeccable manners - for while I waited for her in the Scotsman Hotel (where she and her husband, the cartoonist Gerald Scarfe, were staying), Ms Asher had set off for Scotsman Publications - a reversal of celebrity priorities that verged on the astonishing. Persons of importance - and, more specifically, persons of self-importance - do not come to journalists. Journalists go to them. Cap in hand.

But it only takes a few minutes in Asher’s company to realise that self-importance is not a vice she entertains. Nor volubility. She is calm, composed and smiling, with the sort of charm which would suit a vicar’s wife on garden fête duty.

She is also acutely, shrewdly media-savvy. "There are only two kinds of articles written about me," she sighs. "They’re either: ‘Isn’t she wonderful? How does she do it?’ Or, if I have, as I tend to do, pointed out that I am not superwoman, or a domestic goddess, that I’m a mixture of pluses and minuses, like everyone else, then it’s: ‘Jane comes clean: it’s all a front’." She laughs.

Two years ago, when a Scotsman writer asked her if she got annoyed at being constantly asked about Paul McCartney, she said: "I think it only annoys me that people ask me if it annoys me when I’m asked about him. In fact, it doesn’t annoy me any more. I’m in Zen now, I think. I’m beyond that. There’s nothing you could ask me that I haven’t already been asked, even if it is asking me about asking me about asking me about him to the nth degree. I’ve been there."

So, today, we don’t go there. Instead, she says she can’t quite understand why anyone would expect her to answer really personal questions. "I remember one journalist in particular being baffled as to why I wouldn’t talk about certain things. ‘But you’re not my friend!’ I said. This is a game and they don’t seem to realise that you’re letting out a very little bit of yourself, just enough to get the publicity."

The publicity she is seeking in this particular trade-off is for her new novel, her third, entitled Losing It - an ambitious structure involving five different voices speaking in the first person. The subject matter is equally challenging. Charlie, a fastidious middle-aged barrister, wearily serving out the term of his marriage, falls obsessively in love with Stacey, a brutally inarticulate, hugely fat supermarket check-out girl. Where other novels probing late-onset lust for youthful flesh lavish captivating metaphors on the careless beauty of the young, Charlie’s first encounter with Stacey provokes rather different vocabulary: "I dropped my gaze quickly from the face but was even more unnerved at the sight of the shiny pink folds of flesh continuing downwards in vast Michelin-like coils towards the open neck of a green-checked overall. And that was just the beginning. I went on working my way down the overall in disbelieving fascination. From where the material began at the collar, everything was tension: trussed, straining dollops of flesh, battling to burst free of the huge swathes of green-checked cotton encasing them, pulling at the poppers and oozing from the spaces in between in pale pink polyester-covered bubbles."

Following this scarcely erotic introduction, Charlie slowly abandons every aspect of his life: work, wife and children, financial security and social standing. It requires a big leap of faith on behalf of the reader to find such behaviour plausible.

Asher nods. "You may well be right. I take that as fair comment. But love is an extraordinary thing. I do know one film director, who shall be nameless, who only goes out with very, very big girls. But perhaps I hadn’t shown the reader enough to understand why it happens."

Besides, she says she was more interested in the size-ist aspect of the book than in the mid-life lust crisis. "It was the idea of how somebody’s physical appearance affects not only how they see themselves but the way they are treated, and how that changes when they change. That’s where I started."

She acknowledges that some may find a perennially slim woman writing about morbid obesity either irritating or condescending, but she insists she does so from absolute sympathy. "The way doctors treat the overweight is so dreadful. I wanted to write about that." She also allows Stacey to offer an attempted explanation for why most diets fail.

"Shall I tell you what it’s like being me? You know about your dad being an alcoholic, right? You know how he is with booze. One drink, just one drink, and he’s off. He can’t never have just the one, cos it’s like an illness... And drug addicts, them’s the same. They have to stop right out. And they don’t usually manage that, neither.

"I’m like that with eating. I’m a food junkie. I can’t eat normal just like an alcoholic can’t drink normal but there’s a big fucking difference and I’ll tell you what it is. The difference is what you do about it… The doctor says, ‘Right, just eat a little bit. Not the food you really want, neither - just a little bit of all the food that don’t fill you up and don’t satisfy you and don’t give you the feeling that you need. But just enough to keep your fucking addiction bubbling along nicely.’ See."

In the novel, Stacey has a stomach-stapling operation and loses large amounts of weight, another area which Asher says she finds intriguing. "I can imagine the fascination with changing your body so radically. I’ve never been anorexic or close to it, but I do understand when they say it’s all about a girl trying to take control, because there have been times when I have been very thin, and I kind of know the pleasure of being almost too thin. Into the bone. What is that? It’s bizarre."

It might be fair to say that much of the human behaviour which Asher chooses to depict in her novels is indeed bizarre. Her first novel, The Longing, begins in a cosy upper-middle-class urbane world where a rather smug couple are troubled by their inability to conceive. It then plunges into a Gothic psychodrama of passionate discontent; a snatched infant who is starved to death, a hysterical mother and a memorably mad wife. Rage and unrestrained emotion sweep through the pages, reducing cosy certainties to rubble. Could this possibly indicate a dark and tangled alter-ego for its writer?

Asher smiles. "People are always describing my books as dark, and it does surprise me. I’m never sure how much of that is an objective darkness and how much is a contrast to the public image thing."

She understands exactly how that public image thing works. "I always say, you have to have an image whether you like it or not. When you’re in the public eye, you’ll be given an image. You’ll be narrowed down. And, really, I could do a lot worse than this nonsense of domestic superwoman, which of course is not true. I don’t scrub my own floors. Someone else does. I’m a privileged lady."

Indeed, she always was. Her father was a consultant endocrinologist and her mother a professor of music. She and her brother and sister grew up in prosperous Wimpole Street where her father had his practice, and Jane had her first film role at the age of five, playing a deaf child. "It’s a dream for a child, isn’t it? Getting lots of attention, time off school, fun with grown-ups."

She read out the letters on BBC’s Children’s Hour and played Wendy in Peter Pan when she was 14. She left school after O-levels, and at 17 was the quintessential Swinging Sixties chick. She met Paul McCartney at a Beatles concert in the Albert Hall where she was sent by the Radio Times. He said later: "We spent the evening talking about gravy. I told her she seemed like a nice girl." Jane said: "They couldn’t believe I was a virgin."

A four-year relationship began, spanning 1963 to 1967, the most prolific years for The Beatles. Many of those songs were written in the Ashers’ Wimpole Street home, where McCartney lived for three years, having missed a train to Liverpool one night. Jane’s mother taught him to play the flute.

She talks amiably and very generally about her "very happy" childhood, but will not discuss the early death of her father, who committed suicide when gravely ill. That sort of darkness she leaves to her novel-writing. But it may also explain her impatience with religion. "Ha! God!" she snorts derisively. "He’s making a really good job of things at the moment, isn’t he?"

Curiously, the sunny, Julie Andrews well-pressed persona bestowed upon her by the media has not followed her across the proscenium arch. In the theatre she has not been typecast at all. "Yes, I often play rather unpleasant women," she nods. "It certainly helps with my writing. But devious, nasty characters are more fun. Villains are much more interesting than heroes. It’s sad to admit it, but good people can be the least interesting of all."

Which reminds me, right on cue, about the cakes. Those fabulous fondant fancies in as many strange shapes as her own imagination. How on earth did it all start? Asher shrugs. "It was just a hobby in my teens. The first serious one, I made for my sister’s 18th birthday. But I’m a great starter of things. Not a great finisher. I’ve done evening classes in everything from Russian to dress-making. In fact, it could just as easily have been Jane Asher’s fancy-dress shops."

So Asher the brand is not making a bid for world domination? "Hardly." But there are the cake mixes, the cake tins, the tablecloths… What about bedlinen? She laughs. "With the tablecloths in Debenhams, I suppose you might say I’m creeping towards the bedroom… There! You’ve got your headline. ‘Jane Asher is creeping towards the bedroom.’ Perfect!"

And she smiles that charming, likeable but utterly inscrutable smile. And like every other writer before me, I will have to be content.