Basil the Rat (a.k.a. Rats) - S2-E6
Trivia: Melody Lang, who plays Mrs. Taylor in this episode, is Andrew Sachs' wife.
Trivia: Sybil's friend, Audrey, who Sybil is frequently seen talking to on the telephone, has only one on-screen appearence in 'The Anniversary'. She is seen at the very end, when Sybil has just walked out on Basil.
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."
Basil the Rat (a.k.a. Rats) - S2-E6
Trivia: In some shots of this episode, a real rat was used. These shots were pre-recorded due to the production team having concerns that it could run into the studio audience.
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.