Removing Chatsworth 1928 thermostatic mixer shower cartridge

[RFw3Duwrd7Yu]

Shower seems to have a stuck thermostatic valve alas. I've got the plate and handles off this shower valve, and removed the lock pin (I've put it back in place for the photo), but can't seem to budge the cartridge back out (it does push back into the wall about 1cm, somewhat alarmingly). Is there a good way to de-mineralise / rinse the valve in situ? Or anyone have a suggestion on removing these cartridges for servicing? I can't see any specific place where a wrench could be used, and it's plastic on the inside, so hoping to avoid destroying this whilst trying to remove the cartridge.

 

[blup]

Does this need a box spanner and some serious leverage?

Blup

 

[RFw3Duwrd7Yu]

Fixed! I'm posting a summary here in case anyone else happens to come along with a similar issue.

First, the schematic for this shower valve confirmed for me that the tile backer board was concealing a grub screw (see "shower valve" image below). I used a dremel with a diamond cutting disc to remove some tile and then a sharp knife to cut out more of the backer board to reveal the side of the valve unit.

A word to the engineers designing these units: it is really silly to put a grub screw in the side of a concealed shower value, which is DESIGNED to be behind a wall. A cotter pin or screw accessible from the front would make a lot more sense.

Once that screw was removed the thermostatic cartridge unit pulled out without any issues. However, this revealed that the filter mesh screen at the back had only a small amount of debris, so not the issue causing my problem.

Second step was to remove the filter units which are located above and below the thermostatic cartridge. Each has a mesh at the back. I recommend using a 14mm allen key (which are not easy to find) to remove using the threads on the inside. The outer ring is 25mm but a proprietary shape and can't be removed using a normal 24/25mm socket. The upper unit was clogged with a small thread of what I think was PV tape which had become encased in calcium deposits and balled up. This was enough to clog *only* the hot feed to the thermostatic unit and shower head so cold water could still flow freely, but turning the dial to the left and inviting hot water into the mix shut off completely.

With debris removed from the upper filter and everything back in place the shower works fine!