Triton Opal?

Have looked at what's on this page with interest.
My problem is that the shower on full heat is not delivering reallly hot water. I have turned on the bath taps which has persuaded the low pressure light to come on. Turn off taps and the light goes out. The water pressure microswitch seems to be OK.
I have bought and replaced the heater can so that shouldn't be the fault. It's difficult to be sure but I do detect that the water temperature goes up and down slightly when on full heat, but it's never really hot.
Could it be a faulty thermal cutout? Any ideas would be welcomed.