So this film you've made, Agnes, I say.

It's downbeat, grubby, dark. It's the opposite of chic. Frankly, Agnes, it doesn't much look like a fashion designer's movie. The 73-year-old woman sitting opposite me, her face framed by girlish ringlets, is clearly pleased by this. "Good," she says, smiling.

Agnes b - the person behind the brand agnes b - likes to think, I suspect, that she is not like other fashion designers. Probably all fashion designers like to think they're not like other fashion designers. But Agnes b's otherness is part of her public persona. She's the designer who says she's interested in style not fashion, whose clothes are all simplicity and understatement and sell all over the world (particularly in Japan) and who doesn't bother with capital letters. She's a former soixante-huitarde - a participant in the student and workers' protest movement of 1968 - who became a fashion star, someone who tries to square her liberal leftist beliefs with an industry that's all about high-end capitalism, who hangs around with video artists rather than Beyonce and Jay Z, and who has now made a road movie about child abuse. The film is why we're meeting. I would suggest it is not the kind of film Donatella Versace would make.

The b, by the way, is short for Bourgeois, the surname of her first husband. But she was born Trouble. Decide for yourself which is the more appropriate.

We are sitting in a hotel in Glasgow. Her English is good if at times a little wobbly (I don't try my tres mal French on her). She's in the city to promote her movie My Name Is Hmmm (well, there's no agnes b store nearer than London. Might that change at some point, Agnes? "Maybe"), and because she's good friends with Turner Prize-winning video artist Douglas Gordon, who is always telling her how great his home city is.

Gordon stars in the movie. He's pretty good too, playing a Scottish lorry driver who picks up a French girl (Lou-Lelia Demerliac) fleeing her abusive father. Why, I ask, did she think Gordon could act in the first place? "Because he loves cinema and I love him as a person. He's so natural. He's so strong."

The film itself? Well, you might say it's very French. It contains a talk from philosopher Antonio Negri and a dance sequence featuring white-painted ghostly Butoh dancers. Like I said, it doesn't much look like a fashion designer's movie. It's probably not one for the Cineworld audience. But then its director is not interested in making films for the Cineworld audience, here or at home. "In France they make a lot of comedies about fighting couples," she says. "I never go to see those films."

She's always aime'd the cinema. In the past she's clothed Harvey Keitel in Reservoir Dogs and Uma Thurman and John Travolta in Quentin Tarantino's sequel Pulp Fiction. She is friendly with grungy provocateur Harmony (Spring Breakers) Korine and has helped French filmmakers such as Claire Denis and Gaspar Noe to complete films - Trouble Every Day and Seul Contre Tous respectively. If you haven't seen them, let's just say neither qualifies as a light and breezy comedy.

My Name Is Hmmm doesn't quite qualify as light and breezy either, although it is a little more whimsical than a bare description might suggest. That said, it has the darkest subject at its heart. Why make abuse the emotional core of the film, Agnes? "Because I think right now we start talking about it much more than before," she says. "And I know what I'm talking about."

She sees me looking at her. "It's not my story at all," she quickly qualifies, "but I know what I'm talking about and I'm very concerned by this problem." She then begins to talk about sexual morality and criminality.

"In France, incest is not a crime. It's a delict. It's a fault, but it's not a crime." (As I understand it, this means that it is a civil rather than a criminal offence.) She is appalled by this. There is some talk of making a new law, she says, and rightly so. "It marks you for ever. For ever."

The back story. Agnes b wasn't bourgeois by name until she ran away from home at 17. But she was bourgeois by birth from the start. She grew up in Versailles. Father was a lawyer. Mother was … umm … let's say distant. She was the second of four children. "I was taking care of the others. I was making the food." Because your mother wasn't good at it? "She was very often away."

It was politics not parental neglect, however, that was to become the fault line between her and her parents when she became a teenager. "They were very right-wing and I started to be left-wing when I was 16, 17."

How did that go down? She makes a noise that can't be translated into letters. "Anyway, my mother said, 'She's a good girl but she's a rebel.' She knew I was a rebel."

It didn't take her long to rebel. She became engaged to Christian Bourgeois when she was 16. She was married at 17. He was 30 at the time. She had twins when she was 19 and left Bourgeois when she was 20. "Leaving a husband with two children at that time, you know, it was very badly seen in the families. Very badly seen. So I managed myself. I didn't want to ask anyone for anything."

Would her parents have helped her? "No. My mother was taking something to wash for the children but that's all. They gave me no money."

And money was a problem. For years. As a result she dressed herself in things she picked up at the flea market. And that was the start of her fashion career. "Elle magazine noticed me as someone dressed in a very particular way." The magazine then made her a fashion editor for young people. She was young herself, 21. "And that's where I started putting my finger in fashion."

She had other interests too. This was the 1960s, a time of student revolt, and when 1968 came along she was out on the streets going on demos, being chased by police, talking to philosophers (Negri included). Her children were eight by this time, playing at home because there was no school to go to during les evenements. "My parents were living outside Paris and they were saying, 'You shouldn't stay.' 'I'm OK, it's fine. I love it.' They were shocked because I didn't want to leave Paris."

She is still marked by those heady revolutionary days. She talks about the ideas of Situationist philosopher Guy Debord and points out that even now her company doesn't advertise, "because it's manipulation".

She eventually opened her own shop in 1975 in Les Halles in Paris. She had been working for Cacherel and others but wanted to strike out on her own. "I had married another young guy who wanted me to be happy with my work. We decided to do a small thing in Les Halles which was very cheap at that time. My father lent me some money and we did everything ourselves. We opened the door when we were ready. No big deal. A little dream comes true. It was our day house and we had the night house which was black everywhere. My children were in black and they loved it. Rock and roll music all the time. Music is a cement."

Since that modest beginning she has built a brand with more than 100 stores around the world (she's particularly big in Japan) and is much loved by the likes of Jodie Foster and David Bowie.

You wonder how she squares the contradiction of her politics and her profession. It is perhaps significant that - rather like fashion's other leading leftist designer Miucca Prada - there is a discreet, unfussy simplicity to her designs. "I love to design clothes but I prefer clothes to fashion. Fashion is very ephemeral. I love people to be happy with their clothes and keep them for a long time because I always choose very good materials and good cuts and this has made me famous in Japan and China. I sell clothes made in France in China which is great for me because I want to support my country and we manufacture in Europe."

That sounds like a coals-to-Newcastle idea. Presumably many of her competitors are manufacturing in the Far East. "When you see a dress that costs €20 [£15] it means people get little money to make it. In France we have a problem with unemployment, so I want to support factories in France and Europe and emerging countries. So we manufacture in Morocco and Tunisia too and … comment s'appelle? … Madagascar. I try to do my best to share all the manufacturing. I have an ethic."

Do you know the conditions in all those factories in Africa, I ask her? "Some people go from my company, so it's checked. Always."

She says she doesn't know many people in the fashion world. Jean-Charles de Castelbajac, because they started at the same time, and Azzedine Alaia, but that's about it. Really, she moves more in artistic circles than fashion ones. She's proud of the fact that her photography gallery in Paris was the first to show Martin Parr and Nan Goldin.

"I discovered Ryan McGinley. I love to go out and open my eyes. I was in a party downtown in New York, in a very weird apartment and we started drinking vodka, a plastic bottle of vodka. Crazy. And that's the day I met with Ryan, who is really now famous. I was representing him for eight years."

She's proud that she received France's legion of honour award in 2010. She has a lot of medals, she says. Does she get to wear them? She has one coat with them all on, she says.

What has age taught her, I wonder. The question doesn't seem to interest her. "I can put myself in any age. I remember everything. I have a memory of feelings and visions." When were you happiest then? "I don't know. I can be happy every day. It depends on the moment. It depends on the people I'm with. I have five children and 16 grandchildren, which is enormous. The youngest is five months old. He and his parents are living with me. I take care of him a lot. I think every day is a present."

There must have been difficult times, though? "It has been hard and sometimes you have a chagrin d'amour and you have people around you who have problems - drugs or chagrin d'amour too. But taking care of people, I love it."

And yet she is still working, still designing and now making movies. She says she might even make another. She is not a delegator. She even designs the agnes b stores. "On my passport it says 'stylist'. I like to go to work on the constructions."

Are you a perfectionist, Agnes? "Yeah. Of course. Of course."

You wonder if she is disappointed that subsequent generations haven't been as fired with the same revolutionary zeal as she was back in the 1960s. "I think young people are more selfish. But they know a lot of things through the internet. It's another way to talk."

But, she admits, things aren't as collective as they once were. "But I'm very optimistic anyway. I wait for young people to have a Rousseau."

The rich, she says, have to learn to share. "We have to share everything." But not many rich people agree with you there, Agnes. "No, but that's my point of view and I don't change. I know what it is, having no money."

Like I said, the b comes from Bourgeois. But it's not who she is any more. n

My Name Is Hmmm is out on selected release on October 17.