During the five years from 1978 - 83 an unassuming civil servant who lived in the Muswell Hill area of London became Britain’s worst mass murderer. Dennis Nilsen, who had been a policeman for a short time, lured at least 15 young men back to his flat, murdered them and then disposed of their remains down the sewer. Only when this became blocked was his ghastly secret discovered, and after his lawyers had failed to establish insanity, he was imprisoned for life