The reason for the delay between The Hound of the Baskervilles and starting work on the Casebook series is that Brett and Hardwicke took their characters back to the theatre in The Secret of Sherlock Holmes. It was play in 2 acts, written by Jeremy Paul, that toured between 30 August 1988 and 16 December 1989. In the West End the play was staged at London's Wyndham's Theatre.
The plot differs considerably to the Arthur Conan Doyle short story of the same name. The setting is also different: the short story takes place in Lausanne, Switzerland, not the Lake District of northwest England. Another notable difference is that in the story, Holmes sends Dr. Watson to investigate Lady Frances' disappearance, as he himself is too busy to take the case.
One of the few episodes with genuine narration.
The location used for the Oxford & Lombard Maritime Bank in Pall Mall was the Port of Liverpool Building. Formerly known as the Mersey Docks and Harbour Board Offices, it is one of Liverpool's Three Graces which line the City's Waterfront.
The location used for the hotel was Underscar Manor in Cumbria. At the time of filming it was a disused hotel but has since been converted to a private residence.