IN A Brompton Road bar sit three expensively dressed girls, surrounded by shopping bags. They are decked out from head to toe in well-cut finery, but this year's most fashionable accessory is worn on the tongue. "E's go' a tewwibuw 'abit", they say, in deepest Estuary, as they discuss a friend with a cocaine habit.

Ten years ago they would have been sharing dorms and speaking like royalty. Today they are footloose and consonant-free.

Tamara Beckwith despairs of them. "I would certainly never pretend that I was brought up in Hackney," she has said, and complains that some of her friends from "equally grand families" are prone to such outbursts as - she mimics - "Aawight, Tam, know worra mean, innit?"

Relax, Tamara - when the girls go home normal service will be resumed. They are simply following the new rule: Never say "brown" in town, it's always "bran". These days you don't just change your clothes to suit the occasion, you change your accent too. Sir Roy Strong may have complained long ago about Princess Diana's "common" accent, but now it is the Knightsbridge norm for modish young Sloanes. OK yah-ing is so Eighties.

There is, of course, a long tradition of rich girls slumming it. However, even at the height of proletarian chic, there were limits. Posh punks kept up standards in the Seventies: their artfully ripped bin-liners were worn with a cut-glass accent. The difference today is that Tamara's pals are not making a statement - they're just going with the flow. Speech codes, like dress codes, have been relaxed. And if London's the place to be, London's the accent to speak.

Times have changed since John Wyndham famously observed that the English were "branded on the tongue". But accent does still matter. It's just that today it is more to do with etiquette than origins.

Serious "downgrading" began in the Eighties, among the students who colonised the inner cities, squatting in council flats and opening galleries and vegetarian cafés. And if you walked the walk (in black jeans and Dr Martens), you had to talk the talk too. A new lingo evolved: let's call it Hackney Down. Since then it has become the lingua franca of the low-paid, low-prestige liberal professions, such as teaching and social work. And its influence can be detected in all those now swimming in the modern mainstream, from young actresses to New Labour's new women.

Some trade down ("I want to do whatever common people do," says the sculptress from St Martin's art school in Pulp's song). For others, it's horses for courses - an accent for the office, one for formal occasions and something for the weekend. Tony Blair's accent reportedly changes with the audience. There's the Oxford drawl, the London slanginess, and even the northern idioms - "aye" - which he occasionally adopts when on visits to his Sedgefield constituency. Blair "just wants to be loved", says one of his aides.

Once, upgraders aspired to Received Pronunciation, the traditional "BBC accent". Nowadays being "well spoken" is usually good enough, a more formal or precise variation on a regional accent. Policeman's English is the archetype: the slightly stilted "jobsworth" version of local dialect. Like Ronseal, whose ad features a classic South-East version ("Ronseew. It does igsactly what it says on the tin"), this accent is aspirational, but within limits. It's the voice of the self-made Middle England; doggedly lower-middle class in outlook. The female equivalent has a mildly officious, lisping "telephone voice", with heavily tapped "t's" and whistling "s's".

Sometimes people upgrade out of necessity. The big publishing houses, television news and current affairs departments, and the "quality press", are largely staffed by RP-speakers. RP remains the voice of authority. Former citadels of RP such as the City have fallen, but accents remain segregated: the public schoolboys are brokers and the "barrow boys" are traders.

Generally speaking, flexibility is the name of the game, as shown in This Life, the BBC series about a group of trainee solicitors sharing a house in South London. Miles, the eligible bachelor played by Jack Davenport, is an archetypal product of the upper-middle class: public-school-educated, with a wealthy father who is an eminent lawyer. Around the house, Miles speaks a lazy urban drawl which we might call Clapham Common: informal yet distinctly middle class. As a would-be Jack-the-lad, Miles senses the effete connotations that can attach to a public school accent. This is most noticeable when he discusses "shagging" with his Cockney assistant Joe, and his accent becomes full-blown "Mockney". In conversation with his superiors he reverts to type and becomes nicely-spoken.

This flexibility may be effective, but it signals a big change in the outlook of the upper-middle classes. In the past they wouldn't have had to worry about fitting in with their social inferiors. Now middle class men embrace "working class" male culture because they want to fit in and not draw attention to themselves.

All accents are in a constant state of flux, geographically as well as socially. Paul Coggle's book, Do You Speak Estuary? describes how Estuary English, the milder Thames Valley version of Cockney, has spread across the whole of South East England. (Try finding a young person with a "rural" Norfolk accent in Norwich, or a Sussex accent in Brighton.)

Estuary is popular, he says, because it is seen as being "modern". In parts of London, though, Estuary is mutating into another metropolitan cadence. In places like Streatham and Croydon, the air is filled with "respecs", "know wh'a mean"s and "innit"s. These young urbanites, many of them white, are talking in the accent of black south London - the Ian Wright version of Cockney.

While Estuary sweeps all before it in the South, prejudice about regional accent is alive and well. "Let's face it," said a Home Counties recruitment consultant quoted in a recent report, "people with Scouse accents sound whiny and people with Brummie accents sound stupid." In a survey of British attitudes, Received Pronunciation came out on top, with the industrial accents: Glaswegian, Scouse, Brum and Cockney at the bottom.

Of them all, Brummies probably get the worst of it - no wonder they tend to stay in Birmingham. (Ask a Brummie the difference between a buffalo and a bison, runs the joke, and he'll reply that you can't pee in a buffalo.)

Whether the joke is on the accent or the bathroom habits, the truth is that neither is yet acceptable in "polite" society. But times move on. Perhaps when the Knightsbridge air rings with the sing-song sound of faux-Brummie, Tamara will be using the bison along with the best of them.

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