An 18-year-old actor in the forthcoming Harry Potter film was stabbed to death during a fight outside a London bar. Rob Knox, who plays student Marcus Belby in "Harry Potter And The Half-Blood Prince", due out in November, was killed in a scuffle in the south-eastern suburb of Sidcup. Police said a 21-year-old man had been arrested on suspicion of murder. The fatal stabbing takes to 14 the number of teenagers violently killed in the British capital this year.