Our version of this song was similar to those already posted, but carried a disturbing twist at the end:

I saw your mum
She opened up her legs; I gave her one

It was really was outstanding
We did it on the landing

It really wasn't funny
She did it for the money

It really was fantastic
The baby turned out spastic

It's a bit concerning that the singer is so pleased his offspring is a retard. Still, at least our "not funny / money" version promotes a serious awareness of maternal prostitutes.

The guy in our year who used to sing this the most has since become a Scientologist and now lives on a commune, roasting babies on an open fire (probably).