An 18-year-old actor in the forthcoming Harry Potter film has been 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 today 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. Campaigners and politicians are worried about the spate of teenage killings. In a statement, Knox's parents Colin and Sally said: "Rob was kind and thoughtful and would always help out others. "The life and soul of the party, he was very outgoing, loved sports, and would always strike up a conversation with people. "He was respectful to others and adored by all his family and friends." Film producers Warner Bros said: "We are all shocked and saddened by this news and at this time our sympathies are with his family." A...