just fitted thermostatic mixer shower shower which i bought off ebay brand new and when i turned the left knob on which is the cold water supply knob the water is boiling hot even at coldest setting,after adjusting the right temperature knob it doesnt seem to regulate the temp untill you press the override switch to go really hot only then it goes freezing,i cut a hole in the wall behind to double check i havent put hot and cold feeds on wrong way around but i havent,any ideas,thanks