Years ago, I saw a documentary that chronicled the life of John Cleese, and they included very a very brief clip from this old program. As far as I can tell, this sketch is not included in the DVD (Correction: the reviewer himself reveals in my comments section that the sketch is indeed included in the DVD. Thanks, Bill!). I remember the opening lines of the sketch to this day, and though she's never seen it herself, Mrs. Mosley loves the line delivered by Cleese (which she utters in an imitation of my imitation). When Googled, these lines are always attributed to "Unknown", so I'm guessing this really is obscure. So allow me to help anyone who tries to find the origin of the quote by stating it here. Cleese plays the fourth man with a crazy look in his eye.
(Four normal looking men, all dressed in suits and bowler hats, sit at a table and stare into the camera. All four speak slowly and deliberately)
First Man: I am a Chartered Accountant.
Second Man: I am also a Chartered Accountant.
Third Man: I am a Chartered Accountant, too.
Fourth Man: I am a Gorilla.
(The first three men make an astonished reaction to the fourth, then turn back to the camera)
First Man: We are all Chartered Accountants.
Fourth Man: Except me.