Found it written down after all. (So much for oral tradition)

OH JEMIMA

Oh Jemima look at your uncle Jim.
Someone called him fat so he's trying to get quite thin.
First he feeds on dry toast, then on bread and ale,
Now he's living on monkey nuts and growing a curly tail.

Oh Jemima look at your uncle Jim.
Playing a game of rugby, risking life and limb.
First he tackles the referee, then he joins the scrum.
Now their picking the pieces up, oh there's poor uncles thumb

Oh Jemima look at your uncle Jim.
Learning to drive a motor, looking very grim.
First he jams the brakes on, then he tries reverse.
Now he's driving along in peace in a nice, quiet, safe, slow hearse

Hope this is what you wanted.