It's not immediately obvious but, rather like its Twickenham stable-mate, "The Case of the Frightened Lady", the film mixes a subtle droll comedy with its main suspense-murder story such that some reviewers are unaware. The comedy is almost like an "in joke". The film is a tongue-in-cheek murder-mystery being played for humorous effect.
I began to notice this when the priest is interrogating one of the (murder) suspects, an older aunt, and momentarily plays with a key-chain. "Oh Don't do that!" she says with an exaggerated petulant hand gesture "It gets on my nerves!".
Everybody but the priest, who remains calm and reasoned throughout, is over-playing. The convoluted plot gives multiple opportunities for shock revelations and curious happenings to which the cast members can react extravagantly. It's Agatha Christie with a sense of humour. Something of the tone of "Arsenic and Old Lace" but not as black.
Some other lines confirm the tone: "How about auntie Kate as Widow Twanky?" "Elementary my dear Watson!" spoken by the silly-ass character with the glasses. The priest takes his shoes off to walk quietly. A violent struggle between two men breaks out right in front of him. Now what would his primary concern be? It really made me LOL.
Seen and enjoyed on Talking Pictures, Freeview and Freesat