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The so-called "fake sheikh" - investigative journalist Mazher Mahmood - hit the headlines this week after singer Tulisa Contostavlos's drugs trial, which was triggered by one of his newspaper investigations, collapsed. The judge told the court he thought prosecution witness Mahmood had lied in giving evidence. Mahmood has long been a controversial figure. His investigations, many for News of the World, have exposed serious wrongdoing. But he is accused of provoking people to commit criminal acts, in pursuit of a lurid newspaper headline. Edward Stourton presents. Producer: Smita Patel.