I am also replacing the cart5 in my bristan shower. Once you have removed the temperature control dial by removing the central screw, the tap dial slides off its splined shaft. (mine was very tight). You will now see a ring retained by two screws. Once you have removed these screws, the cartridge will pull out. It may be tight as it seats in a splined hole in the back of the mixer. wiggle it while pulling it with a pair of pliers. Be prepared for escaping water.
Hope this helps.
Doug
July 2008