After much additional research and thought, I have swapped the feed and return connections to the mixer.
Everything now works how you'd expect: feed gets hot, return slowly gets warmer but stays cooler than feed and you can set the temperature dial to whatever value you want and it works. So it...
I bit the bullet and swapped flow and return. The system is now working and responding exactly how it should do. So it appears my flow and return were labeled incorrectly all those years ago and it's be just about getting slightly warm since then. This explains a lot of other issues I've had...
The flow and return pipes were put in and labelled nearly ten years ago. The rest of the connections about 4 years ago.
The flow pipe takes longer to warm up than the return. The return pipe gets hot almost instantly.
There is flow through my UFH if I set the mixing valve temperature to around...
If I pour water into the hot inlet it flows out of the mixed outlet. Regardless of the temperature/mixture setting.
If I pour water into the cold inlet it doesn't flow out of the mixed outlet. Regardless of the temperature/mixture setting.
I've check and unjammed the pump, so that's pumping fine. I've flushed all the UFH pipes, so they're primed and not blocked. I've checked all the motorised valves, and they all appear to activate: you can hear/see them switch as expected. I've also looked at the mixing valve (where the hot feed...