Removing a thermostat valve from a shower..

[sjw1]

Hi all - first post on the site... seems a great place with lots of good advice..
Hoping somebody can help...
I have a shower and earlier on it wouldn't get hot.. I assume this is a faulty/dirty thermostat?...
I've taken the know off and was going to remove and clean the thermostat but am struggling with how to get it out. If I now turn it on it will get how but if I turn it down to cold and then back up it stays cold.
Shower is below - I think it's a Bristan.

I've removed a circlip from the 'stem' where it's marked red below.

How do I remove the thermostat? Assume I also turn the water off. I really don't want to be taking the surround off the wall or loosening anything needlessly - as you can see it's all tiled and the piping is concealed. Is the thermostat the smaller bit that comes out or the larger 'cogged' bit? (the anti scald plastic fitting goes over the larger brass 'cog' and obviously I don't want to damage them.
Hoping someone can help...
Ste

 

[hi-spec plumbing heating]

you've got to strip it right down.
did you try running the shower and turning it to hot then right round to cold a few times as you can sometimes free the thermostatic cartridge from sticking.

 

[sjw1]

you've got to strip it right down.

As in take the plate off? So the large 'cog' doesn't turn to remove?
I don't suppose you know the make of this shower - is it a Bristan? ..and where to get thermostats? Would it be covered under any kind of warranty?
Thanks,
Ste

 

[hi-spec plumbing heating]

need to remove the nut.

 

[sjw1]

Thanks. You mean the large silver one? And then how would the thermostat come out? Assume everything will be normal right handed thread?
Thanks,
Ste