Hudson reed topaz shower cartridge replacement

[Carrie cj]

Hi
I’ve been trying to access the cartridge to replace it of this Hudson reed shower but I can’t remove the chrome collar with nut to access it. Anyone have any ideas? I even bought a new shower because I was told it’s probably furred up with limescale and would never come unscrewed, but now I have the new one I can see that doesn’t unscrew either.
Any help gratefully received! Every video I’ve seen makes it look so easy to unscrew the collar with octagonal nut attached!
Thanks
Carrie

 

[jj4091]

Do you have to remove the e-ring from the shaft first?

 

[Carrie cj]

I’ve tried that too.

 

[jj4091]

Might be better asking here https://www.showerdoc.com/care/contact-us