How can I repair my Triton TS80si Shower?

Our Triton TS80si shower will not work unless turned to high, when the water is so hot we cannot use it.
It will then turn cold and will not heat unless we turn the controls up even further, thus making the water even hotter. My old Triton80 was perfect and I still have it, what do I need to do to repair the new shower. Any advise gratefully received.