The main plot is inspired by the case of Patrick Mahon, who murdered his pregnant lover Emily Kaye near Eastbourne in 1934. He rented a bungalow, murdered Emily Kaye and dismembered her body there, and invited another woman (Ethel Duncan) to spend the Easter weekend with him whilst Emily Kaye's remains were in a locked bedroom in the bungalow.
The 1961 Ford Consul 382 VPU which DI Fellows (Jack Warner) drives was still on the road in 2022 when it was being exhibited at classic car shows.
This film has no main theme, running or incidental music from start to finish.
The film is based on the book "Sleep Long, My Love" of 1959 written by the American Hillary Waugh. The book was reissued in 1962 as "Jigsaw", by Pan Books. Waugh's book is set in Connecticut, USA, and there is no UK connection in the text. Many of the character names were retained in the film, and much of the dialogue. If some of the remarks by Jack Warner's character seem odd, this is the main reason. The film is fairly faithful to the book for the first 70%, but much more detail takes place in the final section of the book. Many of the plot holes in the film are resolved in the book.