When murals by Banksy appeared all over London like a stencilled rash in the summer, there seemed something familiar about the reaction to them. At first, I wasn’t quite sure what exactly. Then it hit me. It was like when Tom Cruise has a curry at an out-of-town restaurant. He was here! That’s him in the evening paper, beside the overcome maître d’. No one’s particularly fussed about what Tom actually gets up to on his nights out. It’s enough that he brings his fame to a formerly undistinguished locale. His presence puts it on the map.

In a way, Banksy is even more electrifying than the Hollywood star. He’s like Cruise’s character Ethan Hunt. He visits the neighbourhood by stealth, or perhaps his people do. It doesn’t seem to be particularly important what he creates while he’s there – a tableau of a cleaner removing a cave painting with a jet washer, a picture of a nimble ibex – though everyone has to believe it’s a bona fide Banksy or the excitement can evaporate before the paint is dry.

If you’re going to get to grips with the phenomenon of Banksy, it would be foolish to overlook the comic-book mystique that surrounds him. He’s a creature of darkness, like Batman. He’s had the opportunity to accrue wealth on the scale of that possessed by the superhero’s reclusive alter ego,