I've mentioned before that there is an earlier film that first had a go at the "Germans capturing the Prime Minister" plot, when he visits a country house near the coast. This is Lawrence Huntingdon's "Warn That Man" where the target is clearly Churchill, though he is never named. There are plot differences with the two later films - "Warn that Man" has a very similar overall storyline to the much later"The Eagle Has Landed", though. Do you think Jack Higgins had seen the earlier film?
In truth, it's not the greatest of films, though certainly a curiosity, and has the great bonus of the wonderful Gordon Harker in a typical role.
Warn That Man (1943)
rgds
Rob
PS, I've just noticed that the IMDB entry for "Went the Day Well" has a comment that apparently
Basil Sydney refused to re-record some dialogue, saying that he was a member of a religious sect whose members were forbidden to look upon their own image! Maybe he just had a train to catch!