I read the other two "reviews" here - the first written by someone who seems to have seen a different film than the one actually in front of his eyes, and the other by someone who doesn't really get one of the major plot points. But, this is the IMDb so what else is new.
I'd never seen or heard of Cloudburst prior to the recent showing on TCM. It's quite a good little film - well directed by Searle, whose work I don't know at all, with a top-notch score by Frank Spencer, a composer I also don't know. Preston is very good, as are the rest of the players, especially the actor who plays the Inspector. The storytelling is compelling, and there's a surprising complexity in Preston's character. Leo Marks, from whose play this was taken, was a fascinating writer and person - as one of the others points out, he really did work as a decoder during the war - and this isn't the only film he wrote where the central character is a decoder - he also wrote Sebastian, in which Dirk Bogarde plays a decoder. And, of course, Marks gave us the brilliant script to Michael Powell's Peeping Tom.
Worth catching if you can find it.