I don't think you really understand my question as your answer - even though you might need more information - is miles away from my solving my problem. Let's leave it like that.
However thanks for your time and for trying.
Thanks for your answer but I'm going away on holiday tomorrow with very limited email access and don't have any more time to try and figure out what's wrong before I go. I ended up speaking to Canon direct this afternoon and they couldn't figure it out either so I'm actually going to give up and live with the problem.