Triton (mersey) thermo cartridge

[richardstephens36]

Hello all

I'm trying to remove the thermostat cartridge on my concealed unit to service or replace it. The unit is a Triton. I believe it is a Triton Mersey but cannot be sure. The cartridge won't pull out despite considerable force, nor will the nut shaped part of the bit behind it budge. Does anyone know whether the nut shaped part of the casing behind it needs to come off for it to come out, or should the cartridge just come out without it coming off ? ( Trying to work out which bit to lean on harder !)

Many thanks
Richard