80 years ago, in 15 September 1940, the Luftwaffe made a gigantic aerial assault on London in the belief that the Royal Air Force was down to its last few fighters. This, they hoped, would be the decisive clash that finished the RAF, and force Britain to the negotiating table or even pave the way for invasion. To mark this anniversary I went to Bentley Priory, the HQ of RAF Fighter Command, with historian Stephen Bungay. He more than anyone I know is able to describe exactly why the RAF won. A tale of technology, leadership, bravery and organisation. A version of this interview appears on History Hit TV.