Before you do anything.... remember that the power supply to this shower is 8.5 kw .. enough to give you a shock and kill you stone dead.
The Mira Zest has 2 screws on the top of the case .. when these are removed the whole front cover witll pull away and give access to the inards.
Before you do anything your fault describes what will happen when the thermal cut out takes over and cuts the supply to the element causing the water to run cold.
The answer may be to simply turn the control dial anti clockwise and then gradually turn it back until the desired temperature is reached.
If that doesn't solve it I would buy a new one myself and then get a plumber to replace it.
http://www.plumbworld.co.uk/mira-electric-showers-1279-0000?filter={%22kw%22:[%228.5%22]}&gclid=COOn_-Xy5cgCFSoEwwodPNoFkA