My thermostatic mixer valve is leaking so I bought a new seal kit and started to take it apart. However it looks like its been leaking for a while and I cannot get the cartridge out of the main housing with my water pump pliers. I’ve order a 45mm socket which should do the job but my question is am likely to ruin the whole thing by putting a bit of leverage into it?

Hopeful bump!

It could be left hand thread. Also try some heat on it.

You tube has some great how to videos, yes even repairing shower valves.
Hopefully your shower is there