Surface Mounted Thermostatic Shower Mixer not shutting off.

I have a Bristan Java Surface Mounted Thermostatic Shower Mixer installed in my shower room at home. It's been used daily for the past 6-7 years with no issues. For about 4 months the mixer has been slightly leaking from the cartridge seals and this week I decided to replace them.
Fortunately I have the original instructions/technical diagrams for the unit and I was able to take it apart easily. I pulled the cartridge out to reveal three very tired seals that, as I guessed, needed replacing. I ordered a seal kit from Bristan which arrived yesterday and I set about replacing them. I introduced the cartridge back into the mixer housing and all seals were once again water tight.
The issue I now have is that in which ever position I rotate the cartridge, it wont shut off the water. The mixing part works fine.
I donít know too much about mixing cartridges but I can see how the mixer works in terms of temperature, I can see the inside of the cartridge moving backwards and forwards as I move the inner spindle letting in hot or cold water. My question is should anything else in the cartridge move, I canít see how the water is shut off. I have a horrible feeling something has seized but I suppose thatís why I am posting here. I have a scan of an exploded drawing if someone could help me?
Any help is greatly appreciated.
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