Faulty Aqualisa shower mixer?

[tgon]

I have a Aqualisa Aquavalve (600 thermostatic)  shower from new in 1992. The coldwater feed is direct from the cold water tank (non-pressurised) in the loft and the hot water direct from the hot water cylinder.
 
The problem is that only hot water comes out the showerhead no matter where l turn the temperature control lever (which is very stiff to turn). It had been getting worse for a long while where even little movements of the temperature control lever would turn the water either very hot or very cold. But now it seems to have 'stuck' on hot.
 
Could this be the mixer cartridge? If so, and l can get the replacement part, is it a relatively simple 'novice' (me!) repair?

Thanks.

[BillyNoMates]

I've had similar problems with a Miras mixer (not sure if that's the same as an Aqualisa ?) and it was indeed the cartridge.

First time I had Miras out and the Engineer advised that the cartridges only last approx. 3 years. Second time I ordered the part and fitted it myself. The Miras website has some detailed diagrams and it was relatively easy. I also recall that the manual had some good diagrams too.