"I don't care much for fashion, I have always been more interested in art, music and cinema," she shrugs apologetically. Although her company is worth more than pounds 100m with 100 shops worldwide, she appears far happier in the role of fairy godmother to the arts than she does as a rival to Miuccia Prada. She runs an art gallery in Paris which has exhibited Gilbert and George and Martin Parr. Such is her standing in the art world, she presented The Turner Prize last year, and her newly-formed film company Love Streams produced the hugely acclaimed Seul Contre Tous, a gritty tale of one man's life in Northern Paris.
Now Love Streams, named after her favourite John Cassavetes film, has a new project, Peau Neuve, an existential buddy movie which opened at the Edinburgh Film Festival. It concerns a man desperate to change his life but uncertain how to go about it. The film, which takes the hero on a bulldozer-driving course in a remote industrial province, won the International Critics Prize at Cannes.
We retire to her suite to talk about it, but not before she dispatches a handsome young lad off to Virgin Megastore. Her son? "My grandson. He's 17," she laughs gleefully. It comes as a shock to learn that this groovy creature is 57.
"Peau Neuve has a universal theme, everyone can relate to that desire for change," she says. "I decided I wanted to be involved with the film because I totally believed in what it had to say, that where you put your energy in life is where you will see the greatest change."
Agnes b has gone through her fair share of life changes. At the last count she had five children by three different fathers, and takes the "b" in her name from her second husband, French publisher Christian Bourgeois. Born Agnes Trouble , her father was a prominent Versailles lawyer. She went to art college and fell in love with a fellow student. By 19, she was married with twins. "And by 21, I had run away with my babies to Paris. My dream was to be a curator at an art gallery, but suddenly I found I had to earn a living. I never wanted a career in fashion, but that's what came along."
She was hired by Elle as a stylist, and subsequently went to work as a designer with Dorothee, then the French answer to Donna Karan. "Those were hard times for me because I was a single parent and I wasn't earning much. But in May 1968 everything changed. I was living in Montparnasse and I used to join in the demonstrations, it was so exciting. I remember I had just paid for a huge lot of groceries with a cheque that I knew was going to bounce, but all work had stopped so I was OK for weeks."
She didn't start her own label until the mid Seventies, and her first shop was an anarchic affair near Les Halles. Back in 1976, customers would write graffiti on the walls and the designer dyed cotton shirts and hung them on a washing line. People would come in and buy off the peg.
"It was only when I had my own shop that I developed the confidence to speak out. Until then I was very shy, and very wary of men. When I was a teenager, I think I felt very vulnerable, like prey, around men. Probably that's why I married so young, to feel protected from that."
Perhaps this acknowledgement that a lot of women lack confidence is the secret of her success - she creates clothes which are empowering without having sharp edges and shoulder pads. Beautiful fabrics, beautiful colours but nothing too obvious. Many of her original designs are still part of her collection today: cardigans with pearl snaps, striped shirts made from the same hard-wearing cotton that rugby teams use and a leather jacket inspired by a Manet portrait.
"I think clothes should give you the confidence to go out and think about other things," she says. "Take my skirts. They come in four different lengths, that way at least one is always `in' and I don't need to worry about which it is. I'm more influenced by culture than by other fashion designers." A quick glance round any of her shops will confirm this - there are film posters celebrating her favourite style icons: Anna Karina, Jean-Luc Godard's muse, Natalie Wood and Ingrid Bergman (she says her famous white shirt was an attempt to recreate the shirt Bergman wore in Notorious).
"Fashion at the moment is particularly boring. I was in New York last week and I wanted to scream. Whatever happened to bad taste? We need more of it." She skips off to her room and emerges with a mauve and fluorescent pink dress. "Look at this, one of mine. It's totally bad taste but I love it." She deplores how everyone in fashion wants to look like everyone else. "It's boring. And it's getting worse. That's why I love women like PJ Harvey and Patti Smith. They have the courage to be different."
She herself doesn't seem to lack boldness. But the French middle classes, for all their pronouncements of free love in films, don't tolerate rebellion.
"My life has not been easy," she says, chain smoking. "I won't put up with compromise, and I won't stay in a situation just for the sake of it. Many women put up with bad things because they don't have confidence to make a choice."
Unlike many designers, Agnes b puts beliefs before profits. The company does not advertise because she thinks advertising is immoral. Her clothes are all made in France to avoid using exploited labour. And each shop, from Tokyo to Notting Hill, has a basket of free condoms on the counter.
When you talk to her about the future though, she starts to look hazy. She talks about the next film, Innocent, which has been directed by a taxi driver whom she befriended 20 years ago, and her new art magazine, Le Point d'Ironie, which she produces with her friend Hans Ulrich-Obrist. "Every month a different artist does an issue, Louise Bourgeois is doing one next!" she exclaims.
That however is as much as she will project ahead. "My company symbol is the lizard - because it's quick and lazy, the way I work." But surely you must have a business plan? A flicker of sadness flits across her face. "I don't try to plan the future, because I happen to know that you can't. I have seen members of my family try this, I have seen people become seriously ill. And, when you go through that, you realise you just have to wake up each day and say to yourself, `today is a present, let's make the most of it'."