Ian: yes, over here 'Bum' means the buttocks. The American usage has also found its way over here, and is often used as a verb meaning 'to scrounge'. The most common usage is in the expression:
"Can I bum a fag" which means "can you spare me a cigarette". The close proximity of two words 'bum' & 'fag' both of which have differing meanings can lead to some confusion when talking to our colonial cousins.

Nigel