If you look closely at the names of the crew in the closing credits (after the cast of characters), you will see certain names have a red letter in them. The letters spell out GOODY CARNE, the name of the "witch" in the execution reenactment scene.
The original 1961 novel has Mark Easterbrook as the chief investigator and does not feature Miss Marple at all.
Nicholas Parsons is no stranger to Miss Marple. He had previously appeared in a Margaret Rutherford Miss Marple film Murder Ahoy as Dr.Crump.
British TV presenter Holly Willoughby briefly appeared in this witch-themed episode as Goody Carne. She later guested on the Midsomer Murders 'The Witches of Angel's Rise' (S22E6).