This relatively obscure Monty-Python-ism is from episode 33 (third season, airdate 11/30/72) of Monty Python's Flying Circus. It is uttered by a number of characters throughout the episode, and is always a non-sequitur. As used on channel, it's something I say when I can't think of anything else to say, or to test for lurking Monty Python fans.
It appears in the sketches "Biggles dictates a letter", "Climbing the north face of Uxbridge Road", "The show so far", and at the very end of the episode in "The news with Richard Baker". There is also a reference to this last appearance in the "Light Entertainment Awards" sketch from episode 39 (third season, airdate 1/18/73), the last episode of that season.