LAURENCE REES: So there is clearly this difference between what Churchill really believes and what he is saying to the British public?

DAVID REYNOLDS: Well, I wouldn’t put it in such a clear cut way, because I think that the problem for historians is always that we can second guess it. We have hindsight and it’s never as clear cut at the time. Churchill’s hunch is that the Germans are not going to be able to invade. It’s a hunch partly built from intelligence and it’s also partly formed by history. I mean, this is a man who believes profoundly in the channel as the great historic barrier between Britain and the continent, and it will work the other way in 1944; he’s very afraid of mounting attack on France for the same reason. So his gut feeling is that Hitler can’t do this. But he doesn’t know it for certain, he’s not sure, so this is his strong belief but not absolute certainty.

So when he talks in these terms to the public about fighting on the beaches it’s not completely dishonest and, in any case, it’s exactly the kind of language that is necessary to rally the public. One of Churchill’s profound gifts as a leader was this sort of resonant sense of the moment and of British history. You can’t imagine Halifax giving those kind of speeches, Halifax would have squirmed at the kind of language and the rhetoric and the extravagance of them, but for Churchill it’s true to himself, to his sense of history, and it seems to resonate with the British public at that moment.

LAURENCE REES: It has been suggested that Churchill was almost deliberately exaggerating the risk in June/July because he felt he couldn't be replaced as long as there was a real risk of invasion, and he felt vulnerable in some respects politically. Is there anything in that do you think?

DAVID REYNOLDS: Well, I wouldn’t put it quite that way. Churchill undoubtedly recognized that he’s in a relatively weak position politically in the summer of 1940. He has been made Prime Minister because Chamberlain has chosen to resign. The Labour Party would not form a coalition government with Chamberlain, and Halifax has, effectively, bowed out of the job. But Neville Chamberlain remains leader of the Conservative Party and Churchill is very conscious that on the Tory back benches there is still a lot of deep suspicion of him as a political turncoat, a maverick and all the rest of it, and he writes to Chamberlain soon after he becomes Prime Minister saying that, to a large extent, 'I’m in your hands'.

And that doesn’t change really until after Chamberlain’s death in the autumn when Churchill becomes party leader. He does it against the wishes of his wife, a die hard Liberal who feels that Winston should still remain, as it were, above party, but Churchill recognises that he has to have a firm party base in order to make sure of his position. But I wouldn’t necessarily link that up to Churchill deliberately exaggerating the threat of invasion. As I say, he doesn’t know. It could happen. After all, given the amazing things that have happened in the last year nobody would bet their bottom dollar on their own certainties. You know, Churchill did not expect the French to be walked over in a matter of weeks, so even if his instinct is that Britain is not going to be invaded, he is not going to be absolutely certain of that.