Britain's most colourful designer is going global. Oliver Bennett met Vivienne Westwood, business supremo
Alternative Queen Mum, amiable eccentric, doyenne of punk, intellectual dilettante: Vivienne Westwood, national treasure and award-winning fashion designer, has occupied several colourful and highly visible roles in public life. She goes to pick up her OBE without any knickers on - the tabloids crow with delight. She calls 13-year-old girls "sexy" - the same tabloids turn apoplectic with rage. But her apparently effortless ability to generate publicity for herself has detracted attention from a far more striking phenomenon: her eponymous, independent business, which has increased its turnover 20-fold in the last two years, is on the verge of transforming itself into something frighteningly similar to the huge couture houses of Europe.

At 56, Westwood is about to open a new flagship store in London's Conduit Street, where she'll sell her "demi-couture" Gold Label alongside the diffusion Red Label and menswear ranges (this is in addition to her four existing shops in London, Leeds and Manchester). An advertising campaign featuring that most recognisable of models, Jerry Hall, will soon be appearing in glossy magazines in Europe and the UK. And, perhaps most tellingly, next year the company will be launching both a range of sportswear and a fragrance. It can't be long before Westwood's signature logo, the instantly recognisable orb, will be lined up alongside the Chanel No 5 and Miss Dior in airport duty-free shops.

The money made with brand extensions; the image maintained by catwalk publicity and glossy advertising; all this smacks of global aspirations, and seems a long way from her days providing outfits for King's Road rubber fetishists and the boys in Malcolm McLaren's house band. But try and get Westwood herself to talk of sales-per-square-foot, or strategies for breaking into the European markets, and you'll be foxed. When we meet, in the presidential-style suite of her otherwise unpretentious Battersea headquarters, she walks in fashionably late and immediately launches into one of her typical, unstoppable, stream-of-consciousness declamations. On this occasion (polling day for the general election, as it happens) her subject is the moral shabbiness of politicians and the need for peace, harmony and proportional representation. "Politicians can't be honest," she rails. "I really believe in devolution or self-government. Peace and co-operation are the things we should focus on. I'm very keen on Europe. If you have some sort of federalism then people do have some sort of autonomy. I also think - oh, people will think I'm a maniac - that there should be a world army to stop aggression."

Westwood's interview technique walks a tightrope between displays of insecurity - "Don't make me embarrassed," she says, when struggling to make a point, "I'm very tired and inarticulate today" - and authority. Ask the wrong question and you feel that, Red Queen-like, she's about to order your immediate decapitation. And those boring business queries? They're are invariably referred to her Italian managing director, Carlo D'Amario, who sits in for much of the interview. From him we learn that in 1995 turnover was pounds 1m, now it is over pounds 20m; that a few years ago Westwood employed around 20 people, now the figure is 65; and that the company's new ads symbolise "tradition and transgression". This could equally well describe Westwood's USP: her knack of providing the fashion-hungry public with High European Culture seen through a warped looking-glass, while herself remaining the lofty epatant in eternal combat with "those who seem to be normal".

The apparently "normal" were of course most recently exercised by Westwood's catwalk use of a 13-year-old girl during London Fashion Week. "Well, she's not a child: she does know how to get babies, I'm sure," says Westwood, arching her Derbyshire vowels like a character from Jo Orton or Alan Bennett. "I don't think I'm corrupting her. Putting these lovely clothes on; feeling your potential as a woman at that age: it's super; uplifting and positive." Yes, she knows that people have had their lives ruined by abuse. "But when a father takes pride in his daughter becoming a sexually attractive woman: I think that's so healthy and human, and why pretend it doesn't exist?"

The girl in question (now 14) will be joining Jerry Hall in the new ads. "Jerry is wonderful: sweet, generous, amenable and clever," beams Westwood. "And she's very good at making people get on with each other. She can manipulate people: she's really got female power." (This is a reference to the Spice Girls, whom she detests: "You couldn't get more downmarket.") Jerry, of course, doesn't come cheap, and the ads - a series of tableaux inspired by Holbein - must be costing the company a fair bit. Spending a lot of money on glossy campaigns seems to sit uneasily with the designer's independent, unworldly ethos; frankly, when business is booming and Westwood achieves so much free publicity without apparently trying, why bother? "That's a point," she says, "but it is a question of catching people's imaginations and making them understand you, and it emphasises the glamour of your product." A woolly answer, but there's no time to pry further: once more she is in verbal free-fall. "I started off in that little shop in World's End which we still have. I'm still in fashion after all this time. I own my own company. I've survived through effort and quality, often without finance. What was so important to me, was to have access to the general public."

The new shop in Conduit Street, over two floors, will offer that access, though mainly to enthusiastic Japanese visitors who are Westwood's key customers. She already sells well to English people outside London, through her stores in Leeds and Manchester - though despite her Derbyshire heritage, these cities are not emotionally significant. "To tell you the truth - and it may not be not good for business - I'm surprised how provincial people are when you get out of London," she whispers. "Some of the things the young people wear up north, that they think are moving and exciting, are a bit naff. Of course, I don't include my clothes in that."

Her general antipathy to England seems odd for someone who has become an emblem of Englishness; a purveyor of eccentric souvenirs. Does that mean she might, as recent rumours suggested, move to Paris and work for a certain French couture house? "I was never asked to do that," she says, somewhat testily. But she does like the idea of life in the French capital. "The French are generally more in touch with ideas," she says. "You don't have to draw a diagram when you explain something to them." And as for the rest of the world? "I think, for them, I have become this kind of tourist attraction," she muses, as if she hadn't thought of it before. So will she, then, become a kind of Laura Ashley for the Nineties, selling all manner of goods worldwide on the back of a perceived Britishness? No - "I don't do bed linen."

Vivienne Westwood clothes are at: 44 Conduit Street, W1 (Gold Label, Red Label and menswear); 6 Davies St, W1 (Gold Label); 2a St Mary's St, Manchester; Liberty, Regent St, W1; Harvey Nichols, Knightsbridge, SW1; World's End, 430 Kings Rd, SW10 (accessories and Red Label).