Seven centuries after Piers Gaveston was beheaded beside the road from Warwick to Kenilworth, he continues to exercise an unhealthy fascination over dissolute Oxford undergraduates, and his name has been linked with both Boris Johnson and David Cameron.

For according to Lord Ashcroft and Isabel Oakeshott’s new book, 

A distinguished Oxford contemporary claims Cameron once took part in an outrageous initiation ceremony at a Piers Gaveston event, involving a dead pig. His extraordinary suggestion is that the future PM inserted a private part of his anatomy into the animal’s mouth.

This anecdote is all too characteristic of the Oxford of the mid-1980s, a period I found myself investigating for my life of Johnson. Various clubs, including the Piers Gaveston Society, took pleasure in shocking respectable society by doing, or claiming to have done, outrageously disgusting things.

Peregrine Worsthorne identified, as one of the faults of the Thatcher period, “bourgeois triumphalism”, displayed by yuppies who had made pots of money but had no idea how to behave. The Piers Gaveston offered rich scope for bourgeois triumphalism, not that its members would have been happy to be called middle-class.

For callow nastiness, it is hard to beat the Oxford journalism of this period, in which ambitious youngsters made up repulsive stories about each other. I do not know whether the pig story is invented, but it sounds exactly the kind of thing which someone who envied Cameron, or thought it funny to pull him down a notch or two, would like to invent.

It is quite hard to believe that the pig anecdote, if true, would only have emerged now. As Cameron rose to high office, someone who had witnessed the event would have yielded to the temptation to amuse people by telling them about it. Even at the time, there would have been rumours.

And the manners of both Cameron and Johnson were better than those of some of their contemporaries. Neither of them wanted to give gratuitous offence to their fellow students, and neither of them got blind drunk and engaged in acts of ostentatious indecency or wilful vandalism.

Johnson, one may note, was the more famous of the two. He became, at the second attempt, President of the Union, was widely known as a star performer who could reduce audiences to helpless laughter, and was looked on by many of his contemporaries as a future Prime Minister.

But he carried into his first job, a traineeship at the Times, one of the unfortunate assumptions of Oxford journalism, namely that strict accuracy was of no importance. In May 1988, he was given a story to write about the discovery of the remains of a palace which had belonged to Edward II.

In order to sex the story up, Johnson informed readers that “this is where the king enjoyed a reign of dissolution with his catamite, Piers Gaveston”. His information turned out to be wrong, indeed invented by himself, and when Johnson failed to show a fitting sense of contrition, the Times sacked him. The whole inglorious story can be found in my book.

One has to say that Gaveston himself would have regarded that as pretty minor affair. He was a Gascon upstart, and showed a capacity for giving gratuitous offence which exceeds that of any other favourite in English history. The barons were so enraged by the power, titles, money and lands which Edward II heaped on Gaveston that on the day of the king’s coronation, 25 February 1308, they said they would boycott the ceremony unless Gaveston was banished.

Edward calmed them down by promising to obey the next Parliament. But during the coronation, Gaveston had the effrontery to wear the jewels which had just been presented to Edward as part of the dowry of his young bride, Isabella, daughter of Philip IV of France. Gaveston, in imperial purple, was the most ostentatiously dressed man in Westminster Abbey.

To Gaveston was given, by the king, not only the queen’s jewellery, but the honour of carrying, on a velvet cushion, the sacred crown of St Edward the Confessor. And at the feast afterwards, which Gaveston was supposed to be organising, the food arrived late, and was inedible. Queen Isabella wrote home to say how horrible the whole occasion had been.

In 1312, the Earl of Lancaster, who had emerged as the leader of the baronial opposition to Gaveston, managed to have him murdered. But Edward’s relations with the barons never recovered, and in 1327 he himself met, as recounted in my latest book, Gimson’s Kings and Queens, a death unpleasant even by the standards of Oxford in the mid-1980s.

Must we wait another seven centuries for the wretched Gaveston’s influence to exhaust itself?

Gimson’s Kings and Queens is published by Square Peg.

The Rise of Boris Johnson is published by Simon and Shuster.

Call me Dave is published by Biteback Publishing.