When it comes to art, London is best known for its galleries, not its graffiti. However, not if photographer Martin Bull has anything to say about it. While newspapers and magazines the world over send their critics to review the latest Damien Hirst show at the Tate Modern, Bull, in turn, is out taking photos of the latest street installations by guerilla art icon Banksy. In three guided tours, Martin Bull documents sixty-five London sites...