The screenplay for this film was co-written by Roy Plomley, who went on to devise the long-running "Desert Islands Discs" radio format.
The film poster promoted the fact that Dr Morelle had risen to fame with the sleuth's appearances in the BBC Home Service radio series "Monday Night at Eight" (he first featured in 1942).
Miss Frayle's quirky car is a Fiat 500 Topolino ('Mickey Mouse' in Italian).
Dial Close Court, Cookham in Berkshire which portrays Barren Tor, was at the time the headquarters for Hammer Film Productions. Following complaints by local people about noise during night shoots, the company moved to nearby Oakley Court.