The first draft for this episode was very different to the final draft. It started with Johnson about to get married to Toni, taking her exes on his stag weekend and afterwards starting to drink again (he's a recovering alcoholic). Mark and Sophie go to Paris for a weekend, coincidently Jeremy and Nancy also go there to keep up the charade of their marriage to fool the Home Office. Toni leaves Johnson and it ends with the news that he has committed suicide. For various reasons this story was scrapped, although lines and other elements from it are in the finished episode.
The scene where Mark and Jeremy go looking for Sophie in the Quantock Hills was actually filmed somewhere in the Chilterns, probably in the Buckinghamshire region since two other locations in that county are used. None of this episode was filmed in the Quantocks, which are 100 miles away from London in Somerset.
The Crown in Chalfont St. Giles, where Mark and Sophie have dinner, was used as the setting for Captain Mainwaring's bank in Batalhadores Heróicos (1971).
Although Peep Show is famously filmed entirely with POV shots, the scene where Mark and Jez are out on the Quantock Hills looking for Sophie begins with a long distance establishing shot from no one's view.
When Jez realizes he's lost, he again references the Moors Murders (hinted at in S2E4, when he drops the name, "Hindley.")