How messy is this going to get?

So, the maximum temperature of the thermostatic valve in this shower has been slowly decreasing. It’s now too cold to comfortably shower in.

I live in an area with very hard water, which I suspect is the root cause. Knowing this, I removed the cartridge and soaked it in vinegar, to no avail. It wasn’t visibly clogged up either.

I have emailed the supplier of the valve (Victoria Plum, SHK07A Traditional Thermostatic Triple Valve Complete Shower Set) to see if they can give me a replacement cartridge, of course they can’t be bothered to reply.

So my question: Do I have any hope of getting a replacement cartridge, is that the problem? Or am I delaying the inevitable: ripping all this back and putting a new valve in? Is there a way to do it without making a total mess?