Stella McCartney, a vegetarian, has it written into her contract at Gucci, the parent company of her fashion label, that she won’t work with leather or fur.

She is vocal in her condemnation of the coats of her friends – Madonna, say, or Gwyneth Paltrow. She has banned hunts from crossing her Worcestershire estate, and worked with People for the Ethical Treatment of Animals on a film about the brutality of animal slaughter. It’s even been reported she won’t sit on a leather chair.

“Nothing dead,” she once said, “ever walks through my door.”

We’ve arranged to meet at a cafe near her London house and I’ve taken care not to upset her: tweed shoes, a canvas bag. At the last minute I remember my belt and dash to the bathroom to remove it. When she arrives the first thing I notice is her cowboy boots, the color of pale calf, slightly battered. They look so much like leather it’s uncanny.

“Yeah, I know,” she says, and tucks them out of sight. They must be the ones she sells – the veggie shoes that have been such a hit. I bend to admire them again, but she’s tucked her feet so far under her stool I can’t reach them. It’s only then it dawns that something dead may, actually, have walked out of her door.

“Oh, these are leather,” I say. “No, wait, these are vintage,” she replies.

Around Ms. McCartney some puzzling contradictions tend to cluster. Paul and Linda spent all those years bringing up her and her siblings as normal kids. And yet it happened anyway: She graduated from Central St. Martins in 1995 a model of pop chick/fashion/vintage/cool.

You’d think she’d do anything to prove she’s more than her name, but she doesn’t seem able to leave it alone. She called the perfume she launched in 2003 Stella because, “It’s the name that my Mum and Dad gave to me so it is very special,” which could be a sweet form of name-dropping.

She cares enough about the environment to ask guests at her wedding (she married Alasdhair Willis, the former publisher of Wallpaper magazine, in 2003) to donate trees instead of gifts. And yet she works in an industry where it is normal to bike round a press pack comprising four cardboard folders containing between them 95 pieces of thick paper.

And, while she speaks in the streetwise half-mockney, half-transatlantic tones – lots of “kind of like’s,” and a sprinkling of swear-words to show she doesn’t care – she is sufficiently removed from reality to ask without a flicker of irony, “Do you have a country house?”

There are those who jump happily on the losses her label made in its first and second years ($5.1 million and $8.5 million respectively), and who see the flourishing of the Chloe brand since she left in 2001 (sales have reportedly risen by 40%) as proof that increases in sales under her tenure were due to her friend and successor as head designer, Phoebe Philo.

But there are others passionate in their support for her – fashion editors – who blame market forces for her recent rockiness and how a label needs time to develop.

After all the talk, all those sheets of paper, there’s a little bit of the Wizard of Oz about this ordinary woman in her cowboy boots in this small cafe. The three of us – her publicist, Stephane, a snake-hipped Frenchman, is here too – sit on stools at a bar by the window.

She is five months pregnant and has a bad back, which may be why her posture is so erect, her hands for the most part clenched in her lap. She is 33 and looks gentler, less sulky, and more wholesome than in pictures. She’s wearing a purple coat with straps and buttons, and seems too self-conscious about her bulge to take it off. “I’m in total denial about buying stuff for being pregnant,” she says, wrapping it closely around her.

“I really do seem to have an issue with it. I go for anything big enough, anything that will fit me. This is, like, two seasons old.” “Autumn/winter ’03,” interjects Stephane. Ms. McCartney: “Exactly. I don’t even know when it is.”

Stephane: “A year ago.”

“A year ago,” repeats Ms. McCartney.

Stephane is here to check no questions are asked about Ms. McCartney’s pregnancy or “her family”(though both are subjects she keeps coming back to herself) and that we keep to Fashion, specifically the range of “performance wear” she is launching for Adidas.

Stephane is an insistent prompter. How does she choose what to put on when she’s not pregnant?

“I go for the things everyone probably goes for. I don’t go massively for what people think when they look at me. I go for comfort and ease and…”

Stephane: “Sexy as well, I think.” Ms. McCartney: “Sexy. Yeah…”

It’s not that Ms. McCartney is defensive. She’s clearly trying to be nice. It’s more that she seems so crippled by how she’ll be perceived, how you might present her, that she can’t relax enough to reflect on a question.

She says “obviously I…” several times, as if you know as much about her as she does. And a lot of her answers are tangled up in things that have been said about her in the past, or might be said about her in future.

When she relaxes a little she allows herself to be more interesting. I tell her that I’ve just been to her shop – “Was that the first time? Naughty girl!” – and that I’d tried on two items: a floaty jersey top that I’d loved and a ludicrous silk bomber jacket that cost more than $1,800 and which I’d put on for a laugh but which felt quite nice on.

Would she say that her clothes are as much for how they feel as how they look? “Definitely. I’ve always done that. I’ve always felt that was important,” she says.

“I think fashion is about psychological, you know, responses to things and that’s part of the job.” She pauses, and then with sudden dryness: “Thank God. You tried on two things that fitted well. It’s rare to hear that in my industry.”

She has reached a stage in her life, she goes on to say, “when I know I have to be true to myself. I think probably part of my upbringing has made me so I can’t function on things that are purely financially based.”

“I base all of my decisions on whether it’s going to be an interesting project and whether it’s going to have validity and when I can talk about it and not actually talk a load of crap.”

Enter Adidas. They approached her about designing for their “old-school” division, “and I thought, ‘How much can I do with three stripes running down a nylon sweatpant?'”

Instead, she suggested she work on their “high-performance” side, using new technical materials.

“I think they’re surprised at how hands-on I’ve been.”

Stephane interrupts: “And the color palette as well. You made a big influence on that.”

Ms. McCartney glances at him and continues. “Women are not educated in what they have to wear technically to enhance what they’re doing. Sneakers are always crap, they’re baby pink or baby blue, they’re like My Little Pony. They’re offensive.”

She is rubbing her face as she talks, and when she takes her hand away there’s a red mark. “I could talk about it until the cows come home,” she says, then goes quiet.

Something changes shortly after this – as if she has let some sort of feeling through her defenses – and Ms. Mc-Cartney starts talking about her mother, who died in 1998.

“She would have loved some of the things I’ve done recently,” she tells me. “She would have loved all my veggie shoes and she would have dug the Adidas thing. She’d have loved the perfume. It’s a bummer. At the weekend I really wanted to call her, talk crap down the phone. I didn’t have anything to say, just sort of babble. She was the classiest woman I know.”

I tell her that a journalist I know once almost fainted when he went to interview her father and that her mother had been sweet to him. She smiles.

“I would expect nothing less from my mother.”

It seems to be all right now to talk about her family, to touch on her daily life. She spends the week in London, and the weekend in Worcestershire where she’s having a garden planted.

At the moment she is “being, like, this perfect role-model pregnant woman. I don’t go out. I’m not drinking.”

She walks or cycles with her dog, a border collie called Red, in Hyde Park. She rides her horse, Flo Jo, as much as she can, but she’s promised her husband that she won’t for the moment.

“If anything happened, I’d feel terrible. I’m not only responsible for myself and the baby. I’ve got Alasdhair to think about. I’ll be back on my horse the moment I’ve squeezed it out.”

She met Mr. Willis at a meeting – his company was pitching for (and won) the contract to design her logo.

She’s close to her siblings, she says – her half-sister, Heather, her older sister, Mary, and younger brother, James. And, though “I would not be so immodest as to say I’m the best auntie in the world,” she is “madly in love” with Mary’s two children. She doesn’t mention Paul’s daughter from his second marriage, to the model Heather Mills.

At the end of the interview she looks tired. It’s late Friday afternoon, but she has more meetings before she can go home. Her sales are reportedly up 65%, her last collection had good reviews, and Robert Polet, the new chief executive of the Gucci group, has given her and the group’s other eponymous label, Alexander McQueen, until 2007 to break even.

As we stand up she wriggles her shoulders to stretch out her back. When she looks at her feet again, your heart goes out.

“The cowboy boots I should have worn,” she says, “are the ones from the last winter show – like, fake leather with canvas in the middle? And then it wouldn’t have come up.”