Well, a lot of films mentioned here are certainly not forgotten and are, indeed, shown on UK tv quite often, but one film that I haven't seen in decades is the fascinating 'Murder in Reverse' a UK film from 1945.
The basic premise of the film has been used since: A man is sent to prison for murder but he claims that the 'victim' is in fact still alive.
Years later, the man is released from prison, and then finds the 'dead' man and kills him!