Hello Aaron,
Thanks for your welcome I've been doing my best to source the joke online and it does seem to be mentioned in 8 Out Of 10 Cats but no idea when! The annoying thing is that I can picture Jimmy doing his opening stand up and saying it!! Unfortunately, I've got 10,000 words still to do and only 3 weeks so there's no time for a DVD binge session...
Fingers crossed someone happens to know the episode!