Trevi Therm A3000 shower, used to be hot now only warm?

I put a Trevi Therm A3000 shower in a new bathroom in my old house about 2 years ago. Its been fine until recently and I've noticed its not as hot as it used to be. The other odd thing I noticed was that I cant have a cold water shower. If I turn the temp knob fully to cold its not freezing cold like it should be.

I've checked the bath taps and others in the house hot is fookin hot, cold is ice cold so its the shower valve.

I had these showers put in my last house and used them for 8 years with not a single problem so there is probably something made a little cheaper inside this new one!

Is there anything I can service in the valve or is it likely a replacement cartridge is needed? which I see are around £100 odd, gulp! I am mechanically competent so could strip, clean and rebuild them but is this likely to be pointless? Will there be something that needs replaced?

I cant find a service kit for them?

Thanks

non return valves stuck open maybe?

is this within the temp adjustment cartridge? So if I strip and regrease it may fix?

I'm not familiar with that shower, but most showers have them built into the tails where the pipes meet the shower body.

Cartridge will be dirty