Can anyone tell me please which series/episode has a scene where a guide is taking a tour round a cavern?

i think its the second series, there is a fansite which has all the locations and the episode they featured in on it.

I know it was in the live stageshow, not sure which TV episode it was in.

It's in the fifth episode of the first series, according to this website:

http://www.leagueofgentlemen.co.uk/newepisodelists1ep5.shtml