"There have been ghosts on the London Underground, sad, harmless spectres whose presence does little more than give a frisson to travelling and boost tourism. But now there's a rash of sightings on the Metropolitan Line and these ghosts are frightening, aggressive and seem to be looking for something. Enter PC Peter Grant junior member of the Metropolitan Police's Special Assessment unit a.k.a. The Folly a.k.a. the only police officers whose official...