Jason references William Shakespeare's innuendo-laden Act III, Scene 2 of "Hamlet" ("Do you think I meant country matters?"), whilst Alexandra quotes the famous "To be, or not to be?" soliloquy from Act III, Scene I.
Alan and Jason discuss Marion: it was established in Department S (1969) that Marion West was the actress wife of Jason, killed in a Chicago plane disaster. It's implied here that they both vied for her affection.
Jason has a rendezvous at the Vienna Central Cemetery, opened on November 1st 1874 and renowned for its many composer graves - hence the episode title, which alludes to the musical structuring known as Theme and Variations.
Director Cyril Frankel opts for some notably skewed-angled shots, presumably in homage to those utilised by the pioneers of German Expressionistic cinema in the 1920s.
Jason and Alan Keeble were close childhood friends in Hong Kong.