When Isabel Radford says "Must the evil that men do live on, or can it remain our secret?" she is alluding to Mark Anthony's funeral speech for Caesar in Shakespeare's play Julius Caesar (Act 3 Scene 2): "The evil that men do lives after them; The good is oft interred with their bones."
Sgt Lewis's warrant card, which he shows to Mrs Preece, is signed by "Chief Constable K McBain", a tribute to the late Kenny McBain, producer of Series 1 and 2.
Isabel Radford says "Accidents will happen, even in the best regulated families." Morse attributes this to Mr. Micawber, while she thinks it was W.C. Fields. Actually, they are both right. W.C. Fields played Mr. Micawber in David Copperfield (1935) and said this line.
By strange coincidence, the sub-plot about the brewery take-over and its site, has unnerving parallels with what Brent Walker seemingly did with its decision to buy Elstree Studios, re-brand it as part of their Goldcrest film division, and then sell off half the prime site studio land for property development.
Appearing in a relatively minor role here, Betty Marsden had previously appeared in the Rank Organisation's "Carry On Camping" as Harriet Potter. By the 2010s, ITV Studios had outright acquired the Rank Organisation's film library, among several other defunct film company libraries, including their run of "Carry On" films. By coincidence, many of the older actors who appeared in the "Endeavour"-"Morse"-"Lewis" trilogy, had previously appeared in various Rank Organisation films, in various roles.