My heater controller was stuck on cold and I couldn't turn it to hot. After disconnecting the controller mechanism I traced the proble to the valve on the heater itself. It's stuck solid on cold. It happaned suddenly and without giving me any warning, ie. it didn't play up at all, was working perfectly than one day it stopped. Does anyone have any idea how to fix it without replacing the heater itself? The bit where the controller arm scews on looks like plastic so I don't want to force it in case it snaps. Any ideas would be appreciated.