In all the attention paid to the 75th anniversary of Dunkirk, there was scarcely a whisper about Winston Churchill's arrival in No 10 a fortnight earlier, something we now take for granted but which was not assured at the time. In fact rather the opposite. Until three days before this seismic moment in our nation's, and the world's, history (shortly after 6.30pm on 10 May 1940), there was no realistic expectation that Churchill would step into Neville Chamberlain's shoes, and several compelling reasons why he should not. How Churchill landed in Downing Street at the last moment, with much greater odds against him than is commonly supposed, is every bit as interesting as the familiar drama of what followed.