Faeces, urine, semen, blood - bodily fluids have long been a staple for avant-garde artistic expression. So much so that even Britain's most user-friendly artist, Antony Gormley, has dabbled in a bit of semen-painting. So it comes as no surprise for Pete Doherty, that self-styled Rimbaud of pop celebrity, to open a vein for something nobler than a chemical hit.