triton amber III elecctrical shower not heating water?

I have a triton amber III electrical shower. All was working fine. The shower is on the 2nd level of my studio office which is located in a building with no central heating or insulation. It get's so cold that the water in my pipes freezes upstairs when the temperature drops below freezing, as is the case now. Well, when i turn on the shower the water now, comes through fine, but even on the maximum heat setting and full power setting, the water just stays cold and does not heat up. Can anyone please suggest what the problem could be, and what part/parts will need replacement. It would be great if a repair service could be suggested for these types of things as well. How difficult would it be to open the unit and replace a part if you have no experience in doing these types of things. If i had to replace the device with a similar model, how would I go about removing the current unit from the shower wall? It looks pretty secure against the shower tiling.
Would appreciate any help with the matter.
Many thanks
Steve