Félicité Du Jeu (DC Stella Goodman) & Daniel Lapaine (Samuel Knight) also worked together on episode 3.4, Ye of Little Faith (2014), of Death in Paradise (2011) as Natasha Thiebert & Paul Bevans respectively, as well as on episode 9.3, Death on the Nile (2004), of Poirot (1989) as Louise Bourget & Tim Allerton respectively.
Trevor Eve (Det Supt Peter Boyd) & Sharon Maughan (Michelle / Elizabeth) also worked together on Kidnap and Ransom (2011) as Dominic King & Jane Wickham respectively.
Trevor Eve (Det Supt Peter Boyd) & Sharon Maughan (Michelle / Elizabeth) are actually married in real life
Although it isn't explicitly mentioned that Stella has been promoted, she is credited as a "DS" (Detective Sergeant) instead of a "DC" (Detective Constable), starting with this episode.