water not keeping hot runs cold during shower?

I,m having the same problems with my shower too.
I think it is a temperature sensor which can be replaced.
Mine has 2 blue wires and an earth wire attached and is located at the bottom, just as the water leaves the shower.
It is fixed with 2 screws and is black and silver.
I'm looking for spare parts online.

ian wrexham
February 2008