Basil the Rat (a.k.a. Rats) - S2-E6
Revealing mistake: When Polly and Manuel are walking down Fawlty Towers drive with Basil the Rat in the box, there is an arch over the front entrance with Wooburn Grange Country Club written on it. This is the name off the building used as the location for the exterior scenes of Fawlty Towers; this is most visible on the DVD.
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.