Banksy lost his street cred the moment he found fame

But as to the supposed " revelation" that Banksy is far from being a man of the people - can that be any real surprise? Many of the great subversive artists of the 20th century, working when the avantgarde really meant something, were from middle-class and even patrician backgrounds. Frankly, you often need a little in the way of financial cushioning to risk real nonconformity.

Not that I think Banksy ever was truly avant-garde; or rather, such credentials as he had were soon mortgaged as he acquired a certain notoriety. To begin with, graffiti art is a field full of anguished young men desperate for some kind of recognition. The archetypal graffiti artist isn't Banksy but the obsessive-compulsive Enzo, who has marked an estimated 250,000 train windows with his simplistic tag.

As soon as "Banksy" became an identifiable artist - and particularly when his work began to appear in book form and be exhibited in galleries - he ceased to have any street cred at all, no matter that he still hung on to his anonymity. By the time his work began being collected by the likes of - gulp! - Brangelina, he was about as "street" as a Tory transport spokesman.

But anyway, having street cred isn't the same as being avant-garde; rather, it's the search by the jaded mainstream for some exciting and new primitivism. To be avant-garde - as the term suggests - is to be out in front of mainstream artists, creating work that through its sheer daring and brio increases the ambit of what may be possible.

Throughout the 20th century, truly avant-garde artists, writers and filmmakers fought a stiff battle against the forces of conformity: their aim was to make it possible to write and paint and make films about previously taboo subjects, principally sex and religion. They succeeded more than they ever could have believed possible, helping to make a culture in which it is now possible for us to experience the most extreme of mediatised experiences, scant few of which are genuinely art.

That has been one downside of the triumph - and subsequent death - of the avant-garde. The other is that while it's possible nowadays to say anything, nobody much is listening any more. Or rather, they're listening to whichever wannabe - such as Banksy - the media have seized on to.