The deceased at the funeral at the beginning of the episode is Harry Rose, who was the underworld connection for Joss Bixby in the third season episode "Ride."
Morse's former lecturer, Jerome Hogg, refers to his student Randall as "a lover of oysters, I'm afraid"; when Jerome Hogg appears in the "Inspector Morse" story "Greeks Bearing Gifts", set over twenty years later, he is clearly revealed as a homosexual, so this reference to oysters is very likely a covert reference to the famous scene in "Spartacus" where Laurence Olivier, attempting to seduce Tony Curtis, tells him that "some people like oysters and others prefer snails".
Morse attends a string quartet recital. The quartet are performing the same Debussy String Quartet Op. 10 that is used in the Inspector Morse episode "The Way Through The Woods" (Series 8 Episode 1).
When giving advice to WPC Trewlove, DI Thursday quotes "No pasaran!" ("They will not pass") from when he was in Cable Street. Adopted from the Spanish Civil War, this was the slogan of British anti-fascists in the October 4 1936 Battle of Cable Street, when a march by Oswald Mosley's anti-semitic Blackshirts were forced to abandon a pro-fascist march in the face of opposition from East-end Londoners. It suggests that Thursday was a young officer on duty that day.
In addition to referencing the famous "did he fire six shots or was it only five?" line from "Dirty Harry" near the end of the segment, the same scene references another linked moment from Don Siegel's film when Cole Matthews holds Joan Thursday as a human shield with a gun to her head. Fred Thursday takes advantage of the discrepancy in height between Cole and Joan to shoot Cole in the upper chest area just below his right shoulder, something which knocks him off his feet and allows Joan to twist free of him and escape unharmed. This parallels the moment in the 1971 film where the villainous Scorpio tries to use a child as a human shield and Harry unhesitatingly shoots him.