Aqualisa Shower Leaking from mixer unit?

I have a Aqualisa shower fed from my hot tank / mains and it leaks when the shower is off from the front of the mixer unit - so I am assuming that the cartridge needs replacing. I don't know what model it is but it has what looks like a grey plastic front to the cartridge which I believe indicates a manual mixer ...

Q1) Do I need to more precisely identify the model?
Q2) Can I fit a thermal regulater cartridge in there as the temperature fluctuates wildly with other devices using water ...