When a member of Oxford's notorious Bullingdon Club hurled a pot plant through a restaurant window, he was maintaining a tradition going back generations. It was what Evelyn Waugh called "the sound of the English county families baying for broken glass".

The Prime Minister, David Cameron, the Chancellor, George Osborne, and the Mayor of London, Boris Johnson, were some of the best-known members of the club – an elite dining society notorious for its drunken excesses. It has a tradition of "omerta", a code of silence, among its members.

Perhaps unsurprisingly then, mystery continues to surround the identity of the undergraduate responsible for that act of vandalism in June 1987. However it seems that The Independent on Sunday is a tiny bit closer to eliminating possible culprits. It has always been known that Mr Johnson was among the party of Bullingdon members on the night of the incident, as was Mr Cameron.

But last week an eyewitness came forward with a tale of how he recalled witnessing a man "with a shock of white blond hair" lob the pot plant. Happily, we are assured it was not the Mayor of London but one who may also have had blond locks.

Paul Wiffen, a fellow Oxford alumnus, felt compelled to speak out after hearing Mr Johnson speak of his "blinding anger at the callousness and selfishness of the rioters".

But last night, Mr Johnson's spokesman, Guto Harri, admitted that the Mayor remembers the name of the Bullingdon hooligan responsible for an act of vandalism similar to those he condemned. Why then, we asked, did Boris not reveal who it was at the time, considering police had interviewed members of the drinking club in an attempt to discover the culprit?

"That's neither here nor there," said Mr Harri. "It is not my job to hand over a witness to the police to something that happened 25 years ago." But, we asked, was that not a conspiracy of silence? He refused to discuss the matter further. But back to the witness.

Mr Wiffen, who was not in the Bullingdon Club but is a graduate of Keble College from a slightly earlier time, recalls visiting college friends in June 1987. "Everybody was celebrating the end of their finals, and we saw the Bullingdon crowd several times that evening, staggering around town completely wrecked out of their heads," he said.

"What I saw quite late on in the evening from a distance was this guy throwing a pot plant through a restaurant window. There were a whole bunch of them, and then the rest of them all ran off, leaving him on his own, and he legged it."

The 52-year-old filmmaker, who is standing as a candidate for the UK Independence Party in the London Assembly elections next year, added: "I don't agree with any of these rioters at all. I don't think there's any excuse for it, but if there is any excuse it's being in a dead-end situation.

"But the Bullingdon crowd had all the privilege in the world. They were at the best university in the world and then behaved like complete thugs, like football hooligans in period costume."

The events of that night have been much dissected since Mr Cameron and Mr Johnson assumed office. Mr Cameron is known to have retired to bed early. Mr Johnson has recounted that he and several others were chased by police through the Botanic Gardens and spent a night in the cells, though others have said that Mr Johnson actually peeled off home too.

There is no suggestion that Mr Johnson was responsible for breaking the window, as his spokesman was only too keen to echo yesterday: "He knows very clearly that he did not do it." Mr Harri dismissed Mr Wiffen's account as being "politically motivated".

Yet whoever did throw the plant pot has evaded justice thanks to Bullingdon members' loyalty. Now it is a matter for the London Mayor as to whether he will name the guilty party.