Middle class is now a dangerously wide category, stretching financially, educationally and aesthetically all the way from footballers' wives to classroom assistants, by way of impoverished dons and City barrowboys.

Yet within the middle class there is one large mental and emotional territory that can be quite easily marked out, and its denizens spotted. It is they who, like graceful sea creatures swaying and changing colour according to conditions, display the most startling economic adaptability. The reason these people so happily flock to Lidl, Aldi and Asda in their Boden deck-shoes is simple: confidence.

What drives these particular organisms to respond so nimbly to changing financial circumstances is the fact that they believe, deeply and comfortably - if not strictly accurately - that they will always be a bit superior to the common herd. They can spell, and so could their parents; they can pick up classical and literary references (though not necessarily bothering to read the books). They were brought up on classic children's stories and do not think Mr Kipling is just a cake; they know which fork to use and when it is OK to eat with fingers. They dress to please themselves, but would wear red braces or a comedy tie only in a spirit of unmistakeable, subtle irony. They like to think they can chat with dukes and dustmen, and are almost impossible to embarrass. They say they are classless, but know that they're quality.

In boom years they enjoyed the fruits of prosperity, shopping at Waitrose, drinking good wine and paying for school fees and interesting holidays, all without much bothering about the bottom line. Now that fuel, food and fees are rocketing and contracts and bonuses in a slump, they seamlessly morph into habits of frugality. Unlike the more nervous, socially uneasy sectors at the top and bottom of the vast middle class, they have absolutely no need to prove themselves by what they buy. “Because I'm worth it!” was always a meaningless slogan to them; they were already so sure of being “worth it” that 2-for-1 bottles of own-brand shampoo will do just as well as one in a fancy box.

Their threadbare sofa and frayed carpet mean bohemian insouciance, not poverty; using the public library and the (best) state schools is a badge of socialist virtue, not social disgrace. In addition, those in their forties were brought up by war-hardened parents and thus regard domestic prudence as quaintly patriotic, an antidote to everything showy, common and nouveau.

They think the most vulgar saying ever attributed to Margaret Thatcher was: “A man who, beyond the age of 26, finds himself on a bus can count himself as a failure.” They happily take the bus to the opera, either heading for the appalling restricted-view seats or after sneakily buying their first copies of The Sun for the Royal Opera House cheap deal. They're so damn confident that they didn't even feel the need to make a joke of it with the newsagent. They are frankly rather terrifying: Waitrose does well to slash its prices.

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