Thank you all, except for Saab.
Dave Martin : Swapping the mousepad should do it. Try using just a blank sheet of white paper or the desk itself.
Sorry if it's not clear from my original post, but this is one of the first things I tried.
Keef - I will probably do just that. I quite fancy a rechargable cordless mouse. Can you get one which recharges from the USB and not from the mains?