Hi
It may well be a faulty thermostat. Is the shower one with automatic temperature control?
The thermostats can go faulty. A new one can be purchased from aqualisa and it is a simple DIY gjob to replace it.
Go to http://www.aqualisa.co.uk/parts-store/
Find your shower model from the drop down list (browse parts by product) and look for the Thermostatic cartridge.
You can also often buy them on eBay at lower price, once you know the part number.
Eddie, February 2009