how do I fix my zuccetti shower mixer leaking/ replace the cartridge?

Same question as some else asked which was not unanswered.

I remove the grub screw but there is no obvious way of getting the handle off without forcing it.

There is no other cover I can find just a chrome /plastic collar behind the handle with a screw driver slot but doesnt look like that will come off before the handle comes off.

How does the handle come off?