Communication Problems (a.k.a. Theft) - S2-E1
Trivia: On the episode "Communication Problems", when Mrs Richards is in the staff office claiming that her money had been stolen, Basil wound her up by suddenly shouting at her. When she banged her head on the shelf, Basil bent down and said, "Is this a piece of your brain?" which is an iconic moment of that episode. However, John Cleese wasn't originally going to put that line in, and had it cut out because he "didn't like it". Andrew Sachs, however, told him to put it back in because he wouldn't regret it, and so he did. Sachs himself claimed on Fawlty Towers: Basil's Best Bits that it was his "one contribution to rescuing the script."
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.