Temperature control om Bristan Cart 5 not working?

I have recently replaced a Bristan Cart 5. For the first few times of turning on and off the temperature control worked fine. Now it remains a constant tepid at best. The hot water feed into the mixer is hot and the nearby hot water tap dispenses hot water. I have refitted the cartridge several times with the same results. It works fine for a short period of time, then when you return to try it later it won't work. I have tried a 2nd replacement cartridge with the same results. I can only assume I'm doing something wrong.

Can anyone help before divorce proceedings commence?