Hi, just hoping I can get a few pointers on ways that I can remove this shower faucet. As water from the shower is coming out cold so got a feeling it needs a new thermostatic cartridge.

should've taken a picture with how far I managed to disassemble it (I know!), however, managed to do the following.
Took the handle off the front, this revealed a valve in the body, couldn't go any further with that.
Took the back plate off, didn't really reveal anything apart from fixing within the wall.
The 3 discs coming out from the plate towards the handle do the following. The one closest to the wall revolves but can't seem to remove it. The next one along just turns it on and off, and can only turn through about 235 degrees. The front one has a small button like thing that I can push in, but doesn't seem to do anything, when I took the front handle off I could see a bit of metal all the way around the inside of ring and pressing the button seems to push this in, but doesn't seem to do anything! The front one doesn't revolve either it's worth mentioning.

Can't find any grub screws or anything else to try to gain entry and I'm worried if I force one of the discs it'll break! Any idea how I can get to the thermostatic cartridge in this shower would be greatly appreciated!

should've taken a picture with how far I managed to disassemble it (I know!), however, managed to do the following.
Took the handle off the front, this revealed a valve in the body, couldn't go any further with that.
Took the back plate off, didn't really reveal anything apart from fixing within the wall.
The 3 discs coming out from the plate towards the handle do the following. The one closest to the wall revolves but can't seem to remove it. The next one along just turns it on and off, and can only turn through about 235 degrees. The front one has a small button like thing that I can push in, but doesn't seem to do anything, when I took the front handle off I could see a bit of metal all the way around the inside of ring and pressing the button seems to push this in, but doesn't seem to do anything! The front one doesn't revolve either it's worth mentioning.

Can't find any grub screws or anything else to try to gain entry and I'm worried if I force one of the discs it'll break! Any idea how I can get to the thermostatic cartridge in this shower would be greatly appreciated!