When Morse follows the franked letter to a law office, the plaque outside the door reads "Vholes, Jaggers, Lightwood, Solicitors". All three are lawyers who play prominent roles in Charles Dickens novels. Vholes in "Bleak House", Jaggers in "Great Expectations" and Lightwood in "Our Mutual Friend".
The poem Morse recites to Thursday whilst waiting for Deare to show up at Blenheim Vale, XVI - (How clear, how lovely bright)" from More Poems, by A. E. Housman - provides the title for The Remorseful Day (2000), the very last episode of the Inspector Morse (1987) series.
A news broadcast which Morse hears on his radio at the start of the story refers to the situation in Rhodesia in late 1966, a year or so after the sensational Unilateral Declaration of Independence (commonly referred to simply as "UDI") in which Rhodesia's government, led by Ian Smith, broke away from Great Britain and affirmed its independence (whilst simultaneously affirming its loyalty to Queen Elizabeth II). This complex matter caused headlines for quite some time, with the British Prime Minister Harold Wilson at one point claiming that it would be settled in "weeks rather than months" and then failing utterly to do that. Smith remained in charge of the country until 1979; from 1980 onward, the country's name was changed to Zimbabwe.
When Endeavour says "When the hurly-burly's done" he's quoting the second witch in Act 1 Scene 1 of Macbeth.
When Chief Superintendent Bright refers to Endeavour's "Four legs good" attitude, he is alluding to Animal Farm by George Orwell.
Colin Dexter: creator of the original Morse series, sitting on a bench chatting during the opening credits.