That policeman was played by David Horovitz who, once played a villain on a Poirot episode. I seem to remember that the only relatives of Jane Marple that Agatha Christie mentioned were her nephew, Raymond West, his artist wife, Joyce, and the girl who married the clergyman. Was her name Stuff?
Does anyone remember the Miss Marple movie with Helen Hayes as Jane and Bette Davis as her unsuspecting friend?