I thoroughly enjoy this series but why, oh why, do the men (not Poirot) keep their hats on indoors? Even Chief Inspector Japp never removes his trilby, even when interviewing suspects in custody. This is quite wrong for the era.
I think that once they have got the hats on, they would mess up their hair by taking them off.
Have a look at the video of Rod Stewart on the roof of Capitol Records singing one of his recent songs 'Love Is' - great record by the way, but once he puts his hat on during the recording, he doesn't take it off again.