Mr. & Mrs. Brian Norris' Ford Popular - S3-E2
Continuity mistake: When Mr. Norris sets out on his expedition, it is April 24. But when we see his diary, the day is the 24th, but the month (on the top of the page) says "October."
Salad Days - S3-E7
Continuity mistake: In the 'Sam Peckinpah's Salad Days' sketch in Series 3, Show 7, the scene suddenly changes from a bright sunny day to a pitch black night - this was apparently due to technical delays, but the effect makes it look like an extension of the 'pretentious director' parody.
Chosen answer: The song you talk of was originally a poem by William Blake called 'Jerusalem'. It speaks of the possibility of Jesus having visited England. The poem has four verses but you only ever hear the Monty Python boys sing the first one which goes, "And did those feet in ancient time/Walk upon England's mountains green/And was the holy Lamb of God/On England's pleasant pastures seen?" If there's any sort of in-joke connected to it's use, I'm not aware of it. It seemed to just be the standard song/hymn they used when a song was needed that wasn't sketch specific. Some of the sketches it appeared in were 'Salvation Fuzz/Church Police', 'Buying a Bed' and 'The Art Gallery Sketch'. Something that may be relevant, though, is that the only one who was present every time it was sung was Eric Idle. Perhaps he just liked it?