This was the first episode to include footage from previous series in the 'previously' recap at the start of the episode. There are scenes from series 2, 3 and 4 in the recap.
Steve and Kate use the callsigns 3-7 and 4-5 in this episode. This is a reference to The Professionals (1977) in which Bodie and Doyle use the same callsigns throughout the series.