How can I fix my Triton Thermo Shower?

I have a Triton Tyne Thermo Mixer shower that is 5 years old. A problem seems to be occurring on an intermittent basis with the first shower of the day sometimes providing only warm water (even though the water temperature in the immersion heater that feeds the shower is very hot). Switching the unit off and on, or adjusting the temperature setting does nothing. Can anyone suggest what is causing this? Should I be looking to replace the shower, or can it be fixed?