Thermal cutout?

Hello Hi -Spec Plumbing & Heating, Re your ? to my shower prob Tues 10th (what is it doing when you turning it on ?
not heating, to hot, etc give abit more info.) the shower is Neptune Solo, power setting knob on = not -, I have to keep temp knob on fixed setting, if I try turning setting to cooler it gets hotter, try turning setting to hotter it gets scalding, have done test removing shower head & turning to max hot, only a dribble, then turning to max cold, full flow.
yours sincerely,
bill turner.