Webb decided to begin his investigation after seeing the 2006 play Frost/Nixon, about another famous television interview between David Frost and former US president Richard Nixon.
He was inspired to create a documentary about the making of the Panorama interview, and discovered the story of the deception already in two books: a history of the Panorama programme and one of Andrew Morton's biographies of Diana.
The book also reveals that, at the time he started work on his own documentary, he was investigating the employer of his wife, Diana Martin, who was then the deputy editor of Panorama.
Ms Martin's father Christopher had in turn produced the 1994 ITV documentary with Jonathan Dimbleby in which Prince Charles confessed to adultery.
Without that programme, says Andy Webb, it is doubtful that Princess Diana would have agreed to talk with Bashir at all.