Hi Andy,
I have the exact same problem and now on the third cartridge. Divorce proceeding imminent !!
My system is supplied wth a twin 3 bar pump. Everytime I take the cartridge out we get about 6 hot showers and then it goes tepid. Remove and replace and hey presto all ok.
I have had a couple of plumbers and spoken to Bristan. The best answer so far is limescale somewhere in the hot feed or elbow.
I hope someone sees this and can help ! PLEASE !
Rob
January 2011