Hi, i've just fixed one of these for which sounds like a simular problem. A tiler removed shower unit from wall, refitted and now low heat.
Fix.
**** Make sure the the power is isolated (off)
Take the cover off (3 screws, 2 top 1 bottom)
Note, cover the plug hole incase these slip of your screwdriver)
If your careful you should not have to remove the ribbon cable
In the middle of the unit you will see a white stabiliser valve spindle (looks like a cog with a lever on it)
Turn this fully clockwise until the lever hits the stop lug.
Turn your temperature control on the front cover to 10
Refit cover
Hope this helps
Jim the Plumber
Jim
January 2008