Gomorrah has been hailed as a classic mafia movie, which lays bare the savagery of the Neapolitan Camorra and how it developed into a political and cultural force. It is based on a bestselling book, whose author, Roberto Saviano, now lives in fear of his life under armed guard. Former Italy correspondent Ed Vulliamy returns to Naples to meet Saviano and witness, first hand, the brutal gang's reign of terror