My hot grips have only worked on low power only since I've had the bike so I had a little peek inside the cotroller, it seems the tctile switches that do the +/- have given up, I can get a new controller for about £26 but I searched for tactile switches, £1.45 for 10! Guess which way I'm going.....
So if anyone else has the same issue I will have 8 switches of the right height spare, they look like a fairly straight forward soldering job.