Apologies for (almost) repeating my post within the same day, but it occurs to me that the question should be more straightfoward.
Has anyone here replaced the cartridge (only) from a thermostatic shower system, and if so is it a trivial or non-trivial DIY activity?
My general preference would be to avoid playing with anyhing that involves water, but if this is a simple unscrew, remove, push in, screw in place type DIY job I'll give it a go.
(We have two water mains at our house, and fortunately the shower that this affects is on the one that affects the fewest of the other taps in the house so if it all goes wrong I can at least turn it off at the mains and still leave us with drinking water!)
Matt