When, in 1956, Nancy Mitford published her essay on "U" and "non-U" language among the upper classes, she provoked a delightful flurry of responses in the press from such figures as Evelyn Waugh, Peter Fleming, Philip Toynbee and Robert Boothby. In refined society, it was agreed, one does not "take" a bath; one "has" a bath. Nor does one ever say "Cheers!" or "mantelpiece", or answer the greeting "How do you do?" with "Very well, thank you." (The correct response is "How do you do?", again.) And while a dull party might count as a "disaster", that word is never, ever, applied to a real disaster, which must be made light of.
Though most of those engaged in these exchanges were themselves upper class, the tone was teasing. This was a kind of anthropological exercise, and the subjects under observation were exotic creatures, like some Pacific cargo-cult society. It was among many signs in the 1950s that the tide was going out for the class elite who had run the country for centuries. The way they spoke was no longer a model for the rest of us: it was ridiculous.
That same outgoing tide did for Harold Macmillan in 1963, when he was mocked out of office as a Victorian anachronism, an Etonian ex-Guardsman with a rod up his behind, a ludicrous moustache and a tendency to pack his Cabinet with chums off the grouse moor.
Ever since, an aristocratic background and manner has been seen as a curse at the top end of British public life, a curse captured in Harold Wilson's branding of Alec Douglas-Home as "the 14th earl", and still evident in 1990, when the posh-spoken, high-Tory Etonian Douglas Hurd stood for the leadership of his party.
In an unseemly piece of social inversion, Hurd found himself wriggling backwards out of his own class, protesting that his father was merely a farmer, and that he had only got into Eton thanks to a scholarship. But it did him no good, as Brixton man John Major skipped ahead of him into Downing Street.
In fact, until recently, no one whom Nancy Mitford would have considered remotely "U" has been prime minister or leader of one of the big parties since Douglas-Home, and only two have been privately educated (Michael Foot and Tony Blair – Foot at a Quaker school).
After half a century, is this now changing? Is it acceptable to be posh again? Has mass culture come around to the view that maybe the upper classes are not twits after all? There are straws in the wind, and perhaps more than that.
Not least of them is Howard Brenton's well-received play at the National Theatre, Never So Good, which looks set to dish the popular caricature of Macmillan in a way that half a dozen thoughtful biographies have failed to do. His Macmillan is the personification of noblesse oblige, weighed down all his life by a sense of responsibility and guilt that derived largely from his class. That he went to Eton, served in the Guards, married well, dressed for dinner and could shoot a hundred birds in a morning seem in the play to be parts of the burden borne by a good man, rather than the hallmarks of outdated privilege. Coming from Brenton, a writer associated with the left, this feels like an absolution: Macmillan's class is no longer a vice, as Peter Cook's savage satire suggested, and it might even be a virtue.
Now look at the London mayoral elections, the biggest political vote in the country for a single individual. Who is leading in the polls but Alexander Boris de Pfeffel Johnson, of Eton, Balliol, the Bullingdon Club, The Daily Telegraph and The Spectator – a character who, if he does not belong in the novels of Waugh, must, then, have stepped out of the pages of Wodehouse?
Johnson doesn't exactly wear his class as a badge of honour (you won't find that famous Bullingdon Club photograph on his campaign website), but nor does he feel obliged to deny it as Douglas Hurd did. So he is both posh and popular, and he is on the brink of power in London.
It could bode well for another figure in that Bullingdon photograph: David Cameron.
Cameron may not be an aristocrat, but his family tree includes plenty of baronets and his godmother is a former lady-in-waiting to the Queen. The Cameron money may be City money (and therefore, perhaps, "trade"), but it is among the oldest City money around.
He went to the same prep school as Princes Andrew and Edward, and from there to Eton and Oxford, and he is married to the daughter of a baronet and a viscountess. It is fair to say that in class terms he could have looked Harold Macmillan (whose family money came from publishing) squarely in the eye.
The two also share a fondness for their own kind. Macmillan once joked: "Mr Attlee had three old Etonians in his Cabinet. I have six. Things are twice as good under the Conservatives." Not surprising that, as Lord Hailsham put it, "there was an element of the dining club, the country house party" in the way he conducted government.
Cameron, too, likes plenty of Etonians and other public schoolboys around him (Oliver Letwin is an old Etonian and George Osborne went to St Paul's), while there is surely a hint of the country house party about that so-called Notting Hill group that was behind his leadership campaign in 2005.
But perhaps, thinking back to the Brenton play, there is something of the other Macmillan in him, too. He has that charm, that ease with people that never appears patronising, which is the hallmark of the Etonian. He is very clever. He doesn't really believe in anything except pragmatic management. He speaks, too, of a desire to serve his country and to do good – noblesse oblige in other terms.
Has the social order altered in his favour? Have the times come around so much that we are ready for another dose of leadership by toffs, after an interval of nearly half a century?
The historian Peter Hennessy will have none of it. "Harold Macmillan was a very, very substantial personality and beside him the likes of Cameron and Johnson are nothing more than soufflé. I can't see any other linkage than that they went to the same school."
Hennessy points to Macmillan's vast hinterland of personal experience in the First World War, and in the Depression in his constituency in Stockton-on-Tees, which drove his determination to make life better for the working class. He also recalls Macmillan's great scholarship and literary knowledge, his religious background, and his sense of irony and detachment.
In comparison, an all-too-serious Cameron puts him in mind of that famous phrase used to describe David Frost, "a man risen without trace".
The former Daily Telegraph editor Charles Moore (an Etonian himself) is more indulgent. He accepts that since the 1960s there has been what he calls a "prohibition" on Etonians leading the Conservative Party, founded on a conviction that "the voters wouldn't wear it", and he is convinced that is no longer an issue. "We are a more classless society and people don't seem as different from one another," he says. "Cameron doesn't look all that different from any other 40-year-old professional man, certainly not as different as Macmillan was from say, Ernest Bevin.
"It helps, too, that the Conservatives so completely lost power in the 1990s. What people objected to most was the idea that power was given to these people on a plate, but Cameron is seen to be working for it."
It is an intriguing paradox, if true, that the classless society of which John Major once spoke might not only open up opportunities that were previously closed to the less well-off, but could also allow a posh person to do the one job from which the posh were barred.
Another historian of the post-war years, David Kynaston, buys into the idea that, as he puts it, "we don't see things through the same class grid that we used to" – largely thanks to the near-abolition of the industrial working class and the arrival of gender and race as social divisions.
But he still detects class forces that can work for or against Cameron. "Posh people are seen by some as peculiarly suited to power because they are disinterested, above graft and corruption, only in it to do good. I feel it in myself when it comes to Cameron: he gives off that sense that he is the sort of officer who would put the welfare of his men first. "But having said that, I still think that if the Tories come unstuck now it will be because people decide they are a bunch of toffs who don't know anything about the real world."
It was that sense which eventually drove Macmillan from power, but was that to do with his class, or just his age? He once joined the audience to see Peter Cook satirise his government, and found the experience as illuminating as it was humiliating. Perhaps David Cameron should go and see Brenton's play; he would learn something from it.
Brian Cathcart writes for the 'New Statesman'