how do you remove a cart 5 thermostatic cartridge?

I have a Bristan shower, fitted with a cart 5 thermostatic catridge. Lately I haven't been getting the normal range of temperature control. It's either mildly warm or a little warmer than that. I want to remove the cartridge to see if any sludge is restricting the movements, but have lost the instructions on how to do this. Can anyone help.