You need to take the temperature control off, then ease of the retaining clip allowing the flow adjuster to be removed. Then unscrew the two screws holding the retaining plate on, and then attempt to ease the cartridge out. I found it easier to allow the water to flow out first after removing the elbow filters. If you have to, you can refit the flow adjuster and retaining clip and then ease it gently off with a broad blade screw driver or putty knife of some sort.
AndyB
September 2008