Triton T80i shower not heating the water as before?

Hi,

I have a Triton T80si 10.5kw electric shower, recently the shower has not been heating the water properly. I have to turn the dial right up to 10 to get the water reasonably warm, previously the water would have been too hot to have a shower at this setting!

A friend of mine who is quite DIY able changed something called the can assembly on the unit but unfortunately this did not resolve the problem. :(

Does anyone have any idea what could be causing the shower to fail to heat the water properly?... your help would be much appreciated.

Kind regards

Sofia