Given the hype about this subject in one of the papers recently I ought to have guessed what was coming from the shape of the grid. But I didn't, and the PDM was - not shocking, but delightful. Despite having all the clues bar one solved and inserted by midnight I remain completely baffled by 15. I know where it must go, but nothing makes sense. Does this mean that everything else is wrong? Anyone willing to provide a hint?