Though Jack's wife is supposed to be older than Mary (the young woman he had an affair with), in actuality, both actresses (Judy Parfitt and Jan Waters who played wife, Anne, and Mary, respectively) were born the same year.
Mike Pratt's character's name in this episode is 'Jeff', and he was also called 'Geoff' in My Partner the Ghost (1969) (aka Randall and Hopkirk; deceased).
Roger Moore and Edward Woodward would later play Sherlock Holmes - Moore in Sherlock Holmes in New York (1976) and Woodward in Hands of a Murderer (1990).