Name is Mike,before you buy a new mixer, (as far as I know you can not get spares for these units, I have tried) I have now repaired 3 of these units, in each case the problem was caused by a blocked hot water inlet, it seems that the frost causes the limescale depositsto break up and seperate from the pipework and bloke the hot water filter. if you need to check the thermostat unit,, remove it from the mixer body, place it under a very hot water tap, you can then see if the thermostat valve opens, if it does, then run it under a cold water tap and you should see it shut,if it works, check the "o" ring seals, replace if required,smear a little fairey liquid on them, and replace the unit,the thermostat unit are quire robust, and designed to work at quite large temp.swings so the frost should not damage them, as I said the most likely cause is a bloked hot water inlet filter, hopes this helps, Mike
Mike Butcher
March 2011