I just fixed the same problem on mine. First remove the covers so that you can check that the hot water supply is piping hot - if it is then the chances are you need to replace the thermostatic cartridge, a plumber told me this is a common problem with these. I bought one from this website:
http://showerdoc.co.uk/shower-spares/triton/triton-lp-care.html
The page above contained the correct spare for mine - the cartridge assembly on the second diagram. It cost around £100.00 but there was next day delivery and the guys on the phone gave me fitting advice. Basically once you have isolated the water supply you just undo 4 screws and ease it out. It's a bit tricky to get the new one in as the o-rings need to seat correctly, took me two attempts! Good Luck!
Rob Neale, January 2009
