Has it always been to powerful/hot ?
Insufficient or low water flow - there should be a thermal cut-out to prevent this anyway.
You could try putting a larger shower gead on the hose - this will allow the water to pass through quicker so not gathering as much heat.
B & Q have a good selection.,
I've just 'repaired' my Mira Sport shower with the same sort of problem. Turned out the 'pilot valve' had to be replaced which cost me �40 for the part (inc p&p) and 45 minutes of my time. A competent DIY'er can usually do the job themselves.
Had two plumbers to look at it who both told me it would be full of limescale and should be replaced at a cost of around �200. Yeah sure! It's working fine now.
I echo point&push. People at showerdoc are so helpful, and the web site has exploded drawings of showers so you can track down the bits you want. Cost us �90 for new internals for an obsolete shower, instead of �400 for a replacement complete mixer unit