Galaxy g8000s shower not providing hot water?

We have the above shower fitted above my bath, this has been fitted roughly 10-13 years ago. This has worked fine as a rule until this week; when i switch the unit on now, the power light does not come on, but the 'overheat increase' light does come on.

When the temp control knob is turned to anywhere in the red area, i.e. asking for hot water, the flow is very slow, and cold, and if turned fully round hardly any flow at all.
when the temp control knob is turned to a cold setting, i.e. anywhere in the blue area, the flow of water is ok.

We have the middle dial set to 'III', it is also very hard to turn this dial now to a 'II' or 'I' setting. I have looked at potential answers on here tonight, is it a solenoid that needs replacing? I have purchased a resistance meter today so will be checking the solenoid when it arrives.

Any help or advice in advance greatly appreciated.