Shower thermostatic cover stuck fast!

[TONK249]

Hi all, I need to remove the thermo cartridge from our shower to de-scale - it has not been taken apart for 13 years. I have taken the hand-wheel off fine. The cover is a 'pull-twist' kept in place with an O ring and does not have a retaining screw, but it is stuck fast and won't budge. I have used (and snapped) a strap wrench, WD40, gone round the edging with a screwdriver to try and break the seal etc etc all to no avail. Help!! Have put a picture in attachment (I think).Any advice much appreciated

 

[Chalked]

Try removing the trim from the front. Might be a circlip. This might then reveal screws to remove the insert.
Post a pic from the front.

 

[TONK249]

Try removing the trim from the front. Might be a circlip. This might then reveal screws to remove the insert.
Post a pic from the front.

Thanks - please see photo from front attached. LB have assured me that the cover is 'twist and pull' which means I have to get it loose. Any suggestions? Hitting (gently) with a wooden mallet??