During his investigations, Morse meets Mrs. Bryce-Morgan, to whose daughter Susan he was once engaged. He gets a glimpse of Susan at the end of the story, although her face is not seen. She is the same character whom Joanna David played (as a middle-aged widow) in "Dead On Time", a famous "Inspector Morse" episode first televised in 1992.
"A lazaretto or lazaret (from Italian: lazzaretto) is a quarantine station for maritime travellers. Lazarets can be ships permanently at anchor, isolated islands, or mainland buildings. In some lazarets, postal items were also disinfected..." - Wikipedia
At one point, Win Thursday says that her daughter Joan has been gone from home for ten weeks. This would place the time period of the story being early September of 1967.
Caroline Bryce-Morgan mentions that her daughter Susan, Morse's ex-fiancee, is now teaching in America at the university of "New Carthage" - the (fictitious) campus which forms the setting of Edward Albee's play "Who's Afraid Of Virginia Woolf?".
When Morse and Caroline are talking, she mentions "Henry" and he mentions "William". Henry is Henry Fallon, the man Susan, Caroline's daughter, married instead of Morse, and William is Susan's older brother, Morse's friend. Both these characters appear in the "Inspector Morse" episode "Dead On Time", set over twenty years later; that story begins with Henry's death, which Morse investigates.