EDIT - really confused now - in the few minutes since posting the reply above (ie. Playing with the mouse to find the lift off distance I have, it seems to be fine again - thought I was messing with the DPI button at the time, trying to switch DPI on the desk and then the mat, to see if it made any difference... Well it does seem to have now - so its got to be the mouse? Or some oddity about how it recalibrates when switching? I dont know enought about it
tbh - but it is a bit odd. Not sure if I want to try and make it not work again, just to be able to reproduce the steps to fix it!