This story has become quite a classic for me, as I've heard the BBC version quite so often. There are some differences with the film version towards the end, but by and large both renditions seem really close. This seems to have been John Paxton's (screenwriter) big break. Nevertheless, I spent quite some effort trying to keep up with some of the more surprising twists in this sinister tale.
I was positively surprised by Dick Powell's performance. I didn't know him before, but his version of Marlowe was full of ambivalence. I've read the behind-the-scene story that the studio and director were strongly opposed to him playing the hard-boiled detective, but I think it is a great role for him. (Chandler seems to have agreed)
The first scene of the film with the interrogation by the police officer had very impressive imagery. Many sinister shadows and the lamp as a blinding spot in the darkness work great.