The Father Christmas Affair - S4-E7
Revealing mistake: At the end, when Mr. Grace is inspecting the Father Christmases, you can see his secretary behind the scenery waiting for her cue.
Revealing mistake: The cake the groom's father has is treated as if its real, but when smashed over Mr. Grainger's head it's obviously Styrofoam and hollow.
Answer: That wasn't possible in 1972. Until recently squatting was a civil offence in the UK, not a criminal one, and removing squatters was a complex and time consuming business.