Deborah Ross talks to JILLY COOPER
So, Jilly, have you decided who you are going to vote for come the election? "The Tories," she replies. Really, I say, you surprise me. I'd have had you down as a Labour voter. "Oh, but I've met John Major and he's lovely, a divine man, and I've met his wife, Norma, and she's just so perfectly sweet."

But Jilly, what about the Conservatives' policies? Shouldn't you be thinking of those? "Oh yes, their policies are terrible, aren't they?" she shudders. "Really ghastly. They've ruined education. And the NHS. They've done nothing for the arts, have they? I may shift, actually. In fact, I think I have shifted already. Yes, I'll vote Labour. Ohhh, look at that doggie. Poor, poor doggie. You can see its ribs. It must be starving." She winds down the window of the car in which we are travelling and blows the passing dog huge, wet mwah-mwah-sounding kisses. Yes, I cringe beside her. Who wouldn't?

Now, I know what you are thinking, because it's what I thought at first. You are thinking Silly Jilly's as silly as ever. And, in part, you'd be right. Jilly Cooper can be hopelessly daft. ("I haven't seen my dogs since Sunday and I'm afraid I'm beginning to twitch," she moans.) But this doesn't mean she isn't smart. It's just that she so wants you to like her.

In the back of the car, which her publishers have laid on for her, are boxes and boxes of Bendicks luxury chocolates. These, it turns out, are to give to fans and PRs and TV hosts plus anyone else she might encounter during this book-plugging day. She has purchased them herself at her own expense. They are very yummy.

Also, she is always incredibly flattering to interviewers. She tells us all we "must write a novel soon." She agrees excitedly with most of what I say, adding "how clever you are" or "how funny" or "what a good question. No, you can't be 35. You only look 24!" Of course, I like her more with each passing minute. I think, possibly, we may even be engaged.

But what is this all about? She is one of the top-selling authors in the country. Her latest book, Appassionata - her sex and Chopin novel - has only just gone into paperback but is already at Number Two in the charts. She is very rich. She owns a whacking great place in Gloucestershire plus a "sweet little house" in Fulham. She can want for nothing, and has nothing to prove to anybody. Or so you might think.

But the trouble with Jilly is that she really does think of herself as rubbish and always has done. Her mother was beautiful, whereas she wasn't and isn't. She disappointed her parents by failing to get into Oxford or Cambridge. "Everyone wants a daughter at Oxford or Cambridge, don't they?" she sighs. Later, after she'd married, she found she could not have children. "And I think if anything makes you feel inadequate, it's that. You tend to think of having children as a biological certainty, so it comes as a terrible shock when you can't." At various points, she refers to herself as "wet", "slow", "stupid" and "fat.". Critics who are nasty cut her to ribbons. Ones who are complimentary "are just having a kind day".

I tell her I've been reading about this disorder called Imagined Ugliness Syndrome whereby perfectly reasonable-looking people go about thinking they are physically repulsive. Perhaps, I suggest, there is also an Imagined Inadequacy Syndrome whereby perfectly adequate people go about thinking they are hopeless. And if there isn't such a syndrome, there should be because it looks as if Jilly suffers from it.

At this, Jilly laughs one of her big, gap-toothed, Wife of Bath laughs then says: "Except, in my case, I don't think there is anything imagined about it." Of course, this might just be affectation, but I don't think so. Plus it explains why she is so soppy about animals. (They love you unconditionally, don't they?) And why she stuck so steadfastly by her husband after his fall from grace. Perhaps she thought she would never do better. Does she like herself?, I ask. "Yes. I mean, I think so. Although I don't like myself when I do mean-spirited things." Like what? "Like bitch behind people's backs. I'll say God that person's a terrible cow. Then they'll call and I'll say: 'Darling, how wonderful to hear from you'." Jilly, that's not mean-spirited. That's normal.

How, I ask, would she describe her novels? "I just write what I write," she says. But does she think they are any good.? "I don't think they're anything, although they sometimes have wizard jokes in them." She knows the literary establishment sneer at her and, yes, she does find it "very hurtful." To cheer her up, I tell her that whatever anyone might say about literary merit, she has probably brought more pleasure to a lot more people than, say, Martin Amis. At this, she gets quite tearful, grabs my hand, and repeats: "Oh, thank you, thank you." I am minded to tell her if she carries on like this I will be forced to call off the engagement.

Mostly, her books feature characters called Perdita or Viking who lurch from set-piece to set-piece via a great deal of improbable sex. Sex has always figured largely with her. She went to an all-girls boarding job where she spent much of her time "longing for boys" and using Gordon More toothpaste as lipstick ("It came out pink"). Of anyone currently living, who would she most like to go to bed with? She thinks about this, then says she doesn't think she could do much better than Colin Firth as Darcy. "The wet shirt!" she cries friskily.

Anyway, after a very exclamatory journey - "Ohh, look, the lilac's out! Ohh, look at that darling doggie!" - we eventually arrive at Bentalls Shopping Centre in Kingston, Surrey, where she is due to sign copies of Appassionata at a branch of Dillons.

She is thrilled to be here, having suddenly remembered she bought her trousseau at Bentalls when she married Leo. "I spent a fortune on my mother's charge card," she recalls excitedly. "I bought a rain-coat." A raincoat? For her wedding night? What was she expecting? "Rain. We went to Norfolk for our honeymoon. I bought a very pretty pink negligee, too. Ohh, I was so happy and in love."

Today, Jilly is wearing an old skirt with mostly eroded buttons down the front, a Laura Ashley white knitted top which has yellowed around the cuffs and a pair of black pumps which, if they are Russell & Bromley as she so claims, are Russell & Bromley from yonks and yonks ago. They are very well worn and worn down. I do not think she is very interested in clothes. Later, she confesses she still owns that negligee and raincoat, and wears them too. Jilly got married in 1961. I am thinking of Jilly in her 35-year-old nightie and mac. I don't think she'd cut it as a heroine in one of her own novels, frankly.

First off, when we arrive, Jilly must rush to the loo so she can spray herself with something lemony from Penhaligon and apply Yardley's Autumn Glow foundation to her cheeks. She has, she says, a very florid complexion and unless she applies her Yardley: "I look like a Dutch cheese coming at you." Her hands are midwifely and the mottled colour of a Blooms salami. Endearingly, there is dirt under her fingernails. "Now, you must write a novel," she repeats again as she sprays and smears and teases the famed hair-do with the exploding fringe. But Jilly, I tell her for the umpteenth time, I don't want to write a novel. I can't write a novel. I wouldn't know how to move characters from room to room. "Oh, that's easy," she exclaims. "You just say: "Rupert went from the sitting room to the kitchen."

She is very kind to the fans who have queued up to meet her, and there are a great deal. Most are plain, Mrs Doyle types with flat hair parted down the middle and then brutally gripped with plastic slides on either side. "How wonderful of you to come," she says, squeezing their hands between hers. She is generous with her time, genuinely grateful they have bothered to turn up. She signs a good many books.

Her father, Bill, was a brigadier in the Army before returning to Ilkley, Yorkshire, where he became managing director of Spooner Ovens. Her mother, Elaine, was divine in every sense. "She was extremely beautiful. Men would come to the house just to gaze at her. She was very dainty, never more than seven stone. Whenever she came to my school I was so proud, because she was always the most beautiful of all the mothers. I wasn't fat as a child, but I was big. I was 11st 3lbs when I went on an exchange to France where I fell in love with the host's son, Michel, but one day he turned to his father and said about me: 'Papa, elle est grosse and laide.'

"My mother never made me feel bad about myself, even though she had to buy a lot of material to make my dresses. I always knew she loved me very much. And we always had such a laugh together. She would call up and say: 'Darling, isn't it wonderful. Virginia Woolf's just won Wimbledon.' "

Although brilliant at English, she was pretty useless at most other subjects at school so, instead of going the university route, she went the secretarial route before coming to London and marrying Leo Cooper, a publisher of military history books, whom she had actually known as a child. His mother had gone to school with her mother. "My first memory of Leo was looking out of my bedroom window in Ilkley and seeing him throw strawberry jelly at a girl who was rabbiting on about how much land her father had. I thought that terribly stylish."

She still doesn't know why she couldn't conceive. A gynaecologist just took a look and told her to adopt. So she did. She adopted Emily and Felix, now in their 30s. No, Emily and Felix have never been interested in tracking down their biological mothers, which she takes as a great compliment. As well she should.

I wonder, though, about her relationship with Leo, and not just because of that business in 1992 when it was revealed he'd had a mistress for six years. It's actually the little things she says that make me squirm. Leo, she says, " is very forthright, so if you've done something wrong he'll tell you." Leo, she continues, doesn't like puddings so "we never have puddings". Leo's never read one of her books. No, that's not true. He read one once when he had flu "but he said it made him feel a lot worse." Shortly after their marriage, he went through her wardrobe and chucked out most of her clothes. She allowed him to do so because "he has much better taste than me."

I wonder if Leo frightens her. Certainly, he seems to. And, yes, he does. "But, then, I think I frighten him too. I think all married couples frighten each other because they have the power to hurt each other, don't they?"

She was very, very hurt by his betrayal, but she didn't leave him and has never slagged him off. Instead, she proved herself loyal and steadfast and carried on churning out the books that support their lifestyle. She does her utmost to paint him in a good light. Leo's never minded her fame. Leo's very fair. "He'll watch England v Scotland at rugby and clap Scotland's tries." Plus, he was right to throw out the things he did. "I owned sarongsters. Terrible girdle-like things which were like rubber chastity belts. They were vile beyond belief. You're too young to remember them." Being only 24? "Being only 24, yes." I think Jilly a very good egg. I just hope her husband knows it.

'Appassionata' is published by Corgi, pounds 6.99.