`If you do not want to dwell with evil-doers`, wrote Richard of Devizes in 1180, `do not live in London`. In her third exploration of the city`s history, Catharine Arnold focuses on the sex trade, from slave girls brought to service Roman troops in first century Londinium, through medieval stews, 18th century sex clubs and Victorian male brothels to infamous `60s call girls and the internet `Belle de Jour`.