Help removing concealed shower thermostatic cartridge

[BiscuitThief]

Hello, does anyone know how to remove the thermostatic cartridge from this concealed shower?

The decorative collar won't budge (tried turning by hand, doesn't seem to be held in by a screw or otherwise).
Not sure I should pull on the cartridge without removing the collar or other parts first.

Product: Vance Concealed Shower, connected to a 1 year old 30kW Main Eco Compact Combi Boiler. The water's temperature is controlled by the bottom lever and the on/off controlled by the top two buttons.

Problem: Hot water turns cold after the first minute or so. If you wait, it rarely turns hot again. If you turn it off and on again it will likely stay cold. It only sometimes gets hot again, and stays hot, if you turn it off for 5-6 mins and turn on again.

We have a sink in the same room which doesn't have this problem.
(https://amzn.eu/d/9ijAqJD, Phoenix Wall Mounted Mixer Tap, "adjust both the water temperature and water flow precisely with the premium ceramic cartridge single control lever.")

Thanks for any help.

 

[tincup]

Turn the water off.
Take the circlip off.
Cartridge should pull out.

 

[Elkato531]

The circlip is not holding the cartridge in, it is holding bits of the cartridge together.
Looks like the cartridge is held in by the M5 grubscrew and the cartridge sleeve needs to be removed to access it.
The sleeve may unscrew or pull off. Try wiggling and/or turning by hand. Rubber gloves can increase grip. Boiling water poured onto the sleeve can help with freeing it.