The following internet site has a PDF of exactly how to do it;
www.browneshealth.co.uk/Thermostatic ​Cartridge.pdf
There's no need to remove the circlip
Thanks anyway
I have a multifunction shower, monsoon head shower, massage jet cabin. The functions are selected from one knob and the water temperature from a knob below the function selector. I have a pressure regulator valve in line before the shower inlets and a 2bar pump supplies the needed pressure...