Hi, there is a very easy way to remove these cartridges,depending on the type of handle you have!
turn water off, remove temp control, this should expose a bronze c clip, remove the clip and the handle, (keep the clip safe!) remove the two grub screws from the front of the valve and take off the silver plate! NOW REPLACE THE BIG FLOW LEVER AND THE C CLIP, WITH A FLAT SCREW DRIVER PLACE IT INBETWEEN THE VALVE BODY AND THE HANDLE THIS WILL MAKE THE CARTRIDGE POP OUT!!!!
bRISTAN SERVICE ENGINEER, May 2009