Hi Pete. I understand your annoyance. Getting the cartridge out of the Bristan Artisan bar-valve shower-mixer is not a well-designed operation.
But here's how.
Just stick it on the floor,
put your foot on it,
then attach a pair of grippers round the teeth that the control knob slips onto,
and then jerk back hard, anti-clockwise.
It might takes half a dozen pulls before it gives.
The new cartridges are made of plastic, so when putting them in make sure you tighten the gripper up hard before screwing the cartridge in so it is flush with the edge of the unit.
G Dawkins
August 2010