Replacing Waltec Shower Cartridge

[Hektor21]

Hi,

I have a leaking Waltec shower and have been trying to replace the cartridge. It is a single tap shower and I have stripped it down as far as I can get. I am not sure which part to remove next. It looks like there may be an outer metal ring or cover and although I have tried using grips to twist it off it won't move even though I have covered it in penetrating fluid. (I cannot see any screws that may be holding this ring/cover in place). I am concerned that if I apply too much force in trying to twist it off I will damage the copper piping in the wall and then I'll have a real problem. I have hopefully attached a picture of how far I have got.

Any advice greatly received.