I have not read the book on which the film is based--but I understand it was a Hercule Poirot book not a Miss Marple book. Anyway in Christie's novels with Miss Marple there was no "Mr Stringer" the real life husband of Dame Margaret Rutherford. But it is Dame Margaret and Stringer Davis that makes the Marple films come alive apart from Ron Goodwin's charming score.
This is the first movie I have seen in which the author of the book on which the film is based is discussed. This movie will not survive trenchant criticism--it is best viewed as a fun movie with loads of British humour.
What strikes you is that the film is really a women's film, with even Robert Morley having to take a back seat to Dame Margaret and Dame Flora Robson who steal the show.
Of the films in the series, 'Murder Ahoy' I thought was most charming because of the supporting cast and a stronger role for Mr Stringer. "Murder at the gallop" has a weaker story and screenplay in comparison. Yet all these Miss Marple films can be enjoyed by one and all, years after they were made.