There's a huge screed of posts of people who have had problems and there are various solutions down to playing around with the registry (which I'd prefer not to do if I can absolutely help it). I've found a new one, (turning off Windows 7 pen and tablet service) and a restart seems to have done the trick for now (although I'm sure I did this some time back and the service seems to have magically restarted again). Oh well, at least I don't have to reinstall the driver.