Me and my big bum

Arabella Weir is one of those women who is used to doing a million things at once. When I meet her in The Charlotte Street Hotel bar she is simultaneously saying goodbye to a young man called Chris, waving manically at me just in case I haven't recognised her, trying to grab hold of a waiter to order her fifth coffee of the morning and mouthing greetings at a sober-suited gent sitting in the foyer who she obviously thinks she knows. Then she notices I'm wearing a hairband round my wrist. "I love that," she says, pointing. "Where did you get it from?" We then go through this "I love that, where did you get it from?" routine for about 10 minutes - - my ring, my skirt, my coat, my scarf, my highlights, my accent.

It's all rather baffling and getting a little tiring when she says, "Hmm, I'm annoying you, aren't I?" and stops. I imagine Weir has always been exhausting. She's the kind of person who seems to be interested in everything. I don't suppose there's ever been a time when she hasn't asked lots of people lots of questions. Part of this may be to do with her job - actors and comedians always soak things up like sponges.

Part of it is probably innate. A fellow alumna of Camden School for Girls remembers "Ari" as being "utterly terrifying. She was very dominant and very funny and very rude to the teachers. You either loved her or you were scared of her. I was scared". I can understand that. Weir's like an over-enthusiastic Labrador. Part of her is bouncy and waggy and please like me. The other is a touch more wary and takes quick peeks over my shoulder regularly as if worried who might turn up next.

I wonder if she ever relaxes? "Not really," she says. "I haven't had the time or the inclination recently because I've been very busy." Over the past few years Weir has become a a household name with her book and newspaper column entitled Does My Bum Look Big In This? The original concept came from her work on The Fast Show, especially her character, Insecure Woman.

The novel, which followed the trials and tribulations of size 14-16 Jackie Pane, seemed to strike a chord with thousands of women. "They wrote to me saying how gratified they were that they weren't the only paranoid ones," says Weir. "What woman hasn't felt terrified of her body when having sex with someone for the first time? You know, that terrible feeling when you think, 'He's looking at my bum. He's going to go off me. He's seen my cellulite.'"

Well, she's very frank and, it has to be said, probably the prototype for her protagonist. "Yes, I used to be very insecure," she says. Jackie Pane is one of the most irritating characters I've ever come across. Her self-questioning is so relentless and negative that I found myself wanting to throw the book at the wall. "I'm sorry you found her irritating," says Weir. "It's fiction and you exaggerate with fiction." But it makes me worry for Weir herself. You cannot write about this type of crippling self-loathing unless you've felt it yourself, surely? "Yes, well, she isn't me," says Weir. "But parts of her are."

The main similarity is, apparently, the obsession with her weight. Weir is not tiny but neither is she a blunderbuss, although it is difficult to tell as she is wearing cleverly enveloping clothes. However, she is right to pinpoint the excruciating tyranny for women - and maybe now men, too - of body fascism.

"When I was younger I would go into a shop and be convinced that the assistant was looking at me and thinking 'poor fat cow'. I would make myself struggle into over-tight size 12 dresses. I was always on a diet. I thought all slim women were more successful and happier than me. I dreamed of turning heads not by my wit and bonhomie but purely by my body. I had no confidence."

It didn't help that her parents made her weight an issue. "I think my father wanted a much slimmer, prettier daughter," she says. "He was considered quite a catch when my mother married him - the brightest and best-looking boy at school, that type of thing - and I always felt I'd let him down." She tells stories about how, after her parents separated and her father, Sir Michael Weir, went to live in New York, he was surrounded by glamorous women. "He was very eligible - a diplomat living in New York who drove a sports car. I'd go and stay with him and feel very dowdy compared to his girlfriends."

Her mother, Alison, was, apparently, very clever and beautiful but, interestingly, put on a lot of weight later in life. She was also not much better at improving Weir's confidence. "She'd refuse to give me potatoes. It was very humiliating. She'd say, in public, 'I'm afraid Arabella can't have potatoes because she's too fat."' It's not surprising that Weir turned into a teenage rebel. "I shudder to think of myself at school now," she says.

But, of course, nothing is ever that simple. Yes I'm sure she was a rebellious dope-smoking pain-in-the-neck who only got two O-levels, which must have been very disappointing for a girl of above-average intelligence. But she was also, curiously, straight. She's never had an overdraft. Not even when she graduated from her drama course, a "subsidiary of Middlesex Poly". Even when the acting jobs failed to come in she still found enough work to keep solvent. "Who, seriously, would have employed me as a classical actress?" she asks. "I felt like a phoney. It took me ages to realise that comedy was what I was good at."

And, yes, she may have dated all sorts of "unsuitable jerks", as she calls them, but she's now very happily ensconced with the lovely sounding Jeremy. They met six years ago when she was 38 and he was 26. He came round to decorate her house. They fell in love and now have two children, Isabella and Archie.

"Some people still go on about the age gap," she says, "but it's very boring. A while back I was doing a radio interview and the first thing the man said was, 'How does your boyfriend feel about you being 12 years older than him?' and I said 'If that's the best question you've got then we might as well finish the interview now.'" She doesn't mean to be brusque, she says, but she can't help it. I wonder if a lot of this defence mechanism stems from her relationship with her mother, which seems to be a fascinating reversal of the norm. Weir tells very funny but also very painful stories about her. "When I was young," she says, "I wanted her to be like other mothers. I'd say, 'What's for dinner?' and she'd say, 'How the f*** should I know?' It seems funny and outrè now but it was difficult when I was nine."

The upshot is that Weir has always mothered her mother - hence the capable side of her character. Her two elder brothers were at boarding school. Her sister, Christina, was younger, slimmer and somehow different. "That's what we think, don't we? That young pretty slim girls don't need to try as hard. I think that's why the burden of being mum fell to me."

When, aged 32, Weir finally realised that this was all a bit odd and damaging, she went into therapy. "It was incredible," she says. "It probably made me able to be successful." She came home one night and said to her mother: "You've made me your mum," and, instead of denying it or defending herself, her mother said, "I know darling and you've been a wonderful mother to me." "I gave up then," says Weir. "I just accept that's how she is.

"She's very childish. I'll bring my friends over and in the middle of the conversation she'll start saying 'bored, bored, bored', until the conversation turns around to herself. I tell her to grow up but it makes no difference. She says, 'but darling I am bored.' All my friends love her. They tell me I wouldn't be me if it hadn't have been for her. And we get on very well now."

Weir also says she gets on with her father, although he's a bit stiff. "She could do with losing some more weight," he said about her, with frightening insensitivity, a few years back. It seems to me that her parents have damaged her terribly. Surely it is ingrained in Weir not to see herself as attractive and worth knowing after a childhood of being told she was unappealing and fat?

"I think school helped because I learned that being witty made you popular." But then she went on to put herself at the mercy of casting directors who'd say things like, "You're reasonably attractive. Lose two stone and we might get you a job." "That was pretty debilitating," she says, "but now I'm relieved. Imagine being Ulrika Jonsson, knowing that your career is based solely on your looks and that, once you've turned 40, that's it. It's different for men. John Sargeant can go on forever with that fish face and those few strands of hair draped over the wrong side of his head. Imagine if women did that."

At times she comes out with so much jollity and joshing about private subjects that I wonder if it's all a big show. There is something of the "look at me. I'm fun, great, happy and successful, and I'm not the quivering overweight freak I used to be" about her. She talks loud and laughs loud and says vaguely shocking things as if to prove a point. The topic of oral sex crops up frequently in our conversation. Then again, it also often occurs in her two further books, Onwards and Upwards and Stupid Cupid.

Both of them feature strong but tainted, slightly paranoid, loud heroines with weird mothers and born-again youthful fathers, a selection of firm friends and unsuitable boyfriends. She is the basis of her chick-lit novels. She embodies the life of the north London working mother, someone who feels slightly overwhelmed and hopeless, who has a variety of amusing and talented friends. She has, in the past, taken this persona and, through it, looked at the issues of weight and beauty, relationships and parenthood.

Her latest foray, Posh Nosh, written with her friend Jon Canter, is a spoof on the traditional cookery show. "I'm amazed no one's done it before," she says. "It's such a ripe area for comedy." In it she plays a Delia-meets-Fanny Cradock character who is married to the obviously gay, louche and posh Richard E Grant character. "I wrote it for him," she says. "I didn't know him well but I begged him to do it."

Weir wants to explode the middleclass myth that we're all marvellous cooks. "All that 'just take some fresh ginger'. I mean, who has fresh ginger hanging around?" she says. "It's unrealistic." She's fighting a one-woman battle against these myths of womanhood, she says.

"If there's one thing I know it's this. No one is happy. Everybody thinks that someone else is having a better life. We all think that the slim, blonde woman with the cute kids and the talent for cooking is happy. She isn't. Even Michelle Pfeiffer isn't happy. Once you realise that, you've unlocked the key to your life."

She pauses. "Well, that and how to eat four chocolate bars without putting on weight."

Posh Nosh is on Tuesday, BBC2, at 9.50pm.