Sienna the queen of fashion? More like a schoolgirl frump

British society has always found new ways to keep the masses down, from the feudal system to Ikea and Big Brother. Now we are one nation divided by a common raincoat. Frightfully, frightfully common, since we're talking Burberry check here - once the uniform of stout shires' matrons, now snugly wrapped around a chav somewhere near you (I spied a Burberry hat on my dustman last week).

British society has always found new ways to keep the masses down, from the feudal system to Ikea and Big Brother. Now we are one nation divided by a common raincoat. Frightfully, frightfully common, since we're talking Burberry check here - once the uniform of stout shires' matrons, now snugly wrapped around a chav somewhere near you (I spied a Burberry hat on my dustman last week).

Fashion has become the social force that cracks the class whip, with badges as distinctive as the old cloth cap. But until quite recently fashion was a harmless affair that merely informed women what length skirt they should be wearing, or that grey was the navy blue of Eastern Germany. And fashion in the last century was largely democratic, in that its vitality usually rose from the underclass, from jazz dives, punk rock and rappers through to surf-boys.

In its purest form, fashion existed for the very young and was a sublimely effective way of sneering at your parents and other authorities. "Do you intend to go out wearing that?" signalled that your outfit was a masterpiece. It's hard not to feel nostalgic for the days when outré clothes could give a Yorkshire miner or retired colonel alike a cardiac arrest. Growing up, as I did, in the twin-set territories of the home counties, you were grateful for rare glimpses of fashion flamboyance. I was sartorially influenced for life when a children's clothing designer called Valerie Goad came to live with her young family in my neighbouring village in the mid-Seventies.

Tall and beautiful with severely bobbed hennaed hair and vermillion lipstick, Valerie teamed sweeping Liberty lawn skirts with striped stockings, cap-sleeved silk shirts, hand-knit cardigans and vertiginous heels. She always looked amazing - part Cruella De Vil, part Kate Greenaway. Many years later I met a London friend of hers who said, "I once crashed my car on the Kings Road because I spied Valerie and she looked so incredible I couldn't take my eyes off her."

But the age of fashion icons as innovators is over. Rest softly Elsa Schiaparelli! Goodbye Mary Quant and our lady Madonna of the cone-breasts. Who is now held up as the ultimate icon? Sienna Miller, that's who.

Now Sienna seems like a perfectly nice sort of girl; doubtless that is why Jude Law is marrying her. In fact, she reminds me of my school-mate Sarah, who also has a mane of tousled blond hair and favours cowboy boots. Which is to say Ms Miller looks exactly like well-heeled, borderline Sloanes of a slightly bohemian disposition have looked for the past 30 years.

Last week she was hailed as the queen of London fashion week and pictured in a pink, stripy Matthew Williamson strapless ball-gown looking exactly like a frumpy Benenden schoolgirl at a hunt ball in the late Eighties. But that's no reason to blame the poor girl for the fashion industry's current creative poverty and call this year's Seventies-inspired boho-peasant look the "Sienna" because it sashays so hard upon, yawn, last year's Seventies-inspired boho-peasant look. The indecent recycling of fashion trends got ever swifter in the past decade, but now we've reached the centre of the cyclone the whole business is inert enough to send anyone into a coma. Can you identify a single heart-stopping fashion trend since 1989?

I have a vast wardrobe, but I've come to realise that it's costume, not fashion, that I adore. Fancy dress is second only to godliness - as a bunch of bishops in gorgeous frocks demonstrate - but "Fashion" is Satan's own handiwork, and he's particularly proud of inventing crochet, velour tracksuits and Voyage. Fashion now only exists to enslave the vulnerable masses, addict them to store cards, then sneer at them. Without fashion's evil schemes there would be no ponchos, no Burberry check, no Ugg boots and no sniggering at Colleen McLoughlin.

This is the catwalk's cruellest dictate: that once clothing leaves its precinct and the pages of glossy magazines, its allure drops by the millisecond. By the time a girl in Liverpool is wearing this month's Vogue's top tips, she's a laughing-stock. Fashion is no longer about the clothes, but the context. Only in the fashion world is it acceptable to deride people for being déclassé.

"Chav" is just a thinly veiled substitute for pleb and it's invariably used to mock the very people who have most slavishly absorbed the stylists' message. Such snobbery is the last refuge of the ideologically exhausted. Fashion - you're so OVER.