The Masonic handshake began in the medieval period as a way of telling if a person was a properly trained stonemason, as only true masons would be taught the handshake. It survived into the modern Masonic period as a way for Freemasons to recognise each other in public.
Looking at the overturned Austin Princess, Gene (Philip Glenister) says, "death of a princess" referencing the ATV drama-documentary broadcast 9 April 1980, Death of a Princess. It was a highly controversial account of young Princess Masha'al and her lover who were executed in Saudi-Arabia in 1978. Both the American and UK broadcasts of the film caused livid protests and strong diplomatic, economic, and political pressure from the Saudis.
Gene (Philip Glenister) says, "In the words of Marlon Brando,'Keep your friends close, but your enemies closer'". This is a reference to the film The Godfather (1972) released in 1972, though it was Al Pacino who spoke the line.
When Ray (Dean Andrews) informs Alex (Keeley Hawes) that Jed's body didn't make it to the mortuary, he says, "the Gypsy creams nicked it". Gypsy creams were a traditional UK biscuit akin to custard creams but with an orange or chocolate filling, which have disappeared from the market in recent years.
When asked what his star sign is Gene (Philip Glenister) says, "Serpico", referring to the US movie, Serpico (1973), released in December 1973 about NYPD officer Frank Serpico. It covers 12 years from 1960-1972 when Serpico, after working his way up the ranks to plain-clothes detective, decides to expose all of the police corruption that he has witnessed. Al Pacino starred as Serpico in this Sidney Lumet film.