Do you keep tomatoes in your fridge? If so, you may never make it into the English upper class. According to Mary Killen, etiquette columnist for The Spectator, she has friends who think tomatoes are terribly vulgar and won’t have them in the house.

Nancy Mitford originally stirred up the class debate in 1954 by providing a glossary of terms used by the U and non-U in her essay The English Aristocracy.

U’s, for example, used a looking glass rather than a mirror, wore spectacles, not glasses, and were rich rather than wealthy. Anyone not au fait with U language — who talked about serviettes rather than napkins, for instance — instantly betrayed themselves, in her view, as not one of us.

An anxious class debate ensued which, as the sniping reports of Prince William’s pals sniggering “doors to manual” whenever Kate Middleton was near (a tribute to her mother’s former career as an air stewardess) demonstrate, hasn’t gone away.

Jilly Cooper suggests the modern-day mark of the non-U is political correctness, but what are the other pitfalls for would-be middle-class princesses?

Killen suggests that today a gentleman never pushes a trolley in a supermarket, he always uses a basket. The upper classes also never eat between meals or refer to them as such; it is always breakfast, lunch or dinner.

But it’s also not what you say but the way that you say it. Butter, for example, is “orf”, never rancid, and the Rothschilds have a very nice “hice” rather than home.

AA Gill agrees. Our obsession with U and non-U words is, he insists, an entirely middle-class affectation and as soon as a word is recognised as U, it automatically becomes non-U. Toilet was a very smart word for the Edwardians, as it came from the French. It went out of fashion, though, when their servants adopted it.

It’s your accent that really marks you out, and even that is changing. “When you compare how the Queen spoke at 18 to the way she does now, she has a very standard upper-middle-class accent these days,” says Gill.

As the vulgarity of the tomato demonstrates, the contents of your fridge can also prove a minefield of potentially embarrassing gaffes. The upper class, for example, will only eat runny home-made marmalade, never glutinous or heaven forbid, with “no bits”.

“The ebb and flow of snobbery,” says Gill, “is best exemplified by the pineapple.” Once the ultimate symbol of belonging to the upper echelons of society (King Charles II was even painted holding one), as soon as it was put into tins it lost its social cachet. It now appears as the topping to Hawaiian pizza, “the most common thing you could eat”, he says.

One of the oldest gaffes you can make when rubbing shoulders with the U is being flash with your cash. You are, after all, far more likely to see the Queen at the wheel of a mud-spattered Land Rover Defender than in a sparkling Porsche Cayenne. And walk down London’s King’s Road, the mecca of the original Sloane Ranger, and you won’t see bona-fide members of the U wearing designer labels. In fact they are far more likely to be seen in holey tracksuit bottoms, with unwashed hair and bare feet. Kate may well have been a bit too well-groomed.

She may also have worn a little too much black. Apparently an elderly couple who live in the same Berkshire village as the Middletons refused to sell them their home because Carole wore black. This indicated the family would bring down the tone of the village. How oddly prophetic.

An oft-cited class indicator is grandmother snobbery, going up from “nan” to “grandma” through to “granny” and, in Wills’s case, “Your Majesty”.

According to Gill, the whole class debate was invented by the middle class and the media and the thing that the upper and lower classes have in common is that neither cares about it. None of this is demonstrated more clearly than the rumour that the Queen is rather partial to Coronation Street.

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