There are lots of reasons why you should watch this film (and it repays repeated viewings)but Dirk Bogarde is possibly the best of them. Here he is in a role that few actors - at the time - would have dared attempt (the fact that Bogarde was himself a homosexual, albeit one who preferred to keep his private life to himself only emphasises how much he stood to lose in a less 'liberal' social climate than the one we have today). With one stroke, he ended his increasingly uncomfortable career as a matinee idol and became the great actor he was always destined to be.
This film is criticised nowadays for its attitude to homosexuals: it treats them as sad, tortured creatures who deserve our pity - and whilst I can understand people making that objection, I think that director Basil Dearden and all involved had their hearts in the right place. You'll also see a very early (and moving) performance by Peter McEnery and a couple of excellent acting turns by Sylvia Syms and Dennis Price.
Definitely a film to watch, learn from and savour.