The scene in episode 1 when Paul Brittan makes Sadie tea, asking about the specific order is something he does/did as a psychological test.
Also in 2021, Eddie Marsan and Niamh Algar also appeared in the film Wrath Of Man.
In episode 1 set in 1992, Sadie is handed a post it note with a telephone number with the dialling prefix 0171. 0171 did not come into operation until April 1995.
Anachronism: In the last scene of episode 1, the killer sits down in his kichen and looks at a Polaroid picture of Lizzie. Behind him, mounted on the wall is a Worcester Greenstar boiler, this wasn't released until the early 00s.