41 minutes ago, Adrian Clark said:

Thanks everyone, much appreciated, in response:

Situation.

Andeh. I too wonder if the scree is too thick, however, on a couple of occasions in the past the water temp has gone into the upper 30s and tiles have felt warm (there are no installation records with the house documents).

Radiators (just two upstairs) running efficiently.

Last photo, DP miller, from under white cap.

Second photo. GAC Pin under tcv is free and springs back.

Third photo. Manifold valve pins all free and spring back. A bead of water came through the middle valve when depressed (see photo).

Fourth photo. Flow meters 1.5 and responsive when pump switched from II to III.

Update. Water temp consistently 28 deg.

DP Miller, the boiler cycle is 1 min to bring from 40 degrees to 49.5 degrees and then 9 mins to drop back to 40. The pressure valve is in the green and just over 1 bar.

Welsh Nick. I'll buy a clamp-on IR thermometer and a volt checker. The boiler temp has been on min (40) for three days now (good luck against Japan).

Arigato.

2 hours ago, Adrian Clark said:

the boiler cycle is 1 min to bring from 40 degrees to 49.5 degrees and then 9 mins to drop back to 40

Sounds like the boiler is short cycling. You don't have enough of the heating open for the boiler to push heat into.

10 hours ago, Nickfromwales said:

Turned out to be the boiler flow temp was a little too high, causing the safety temp probe to shut the UFH manifold pump off; then it cooled down and came back on, then it got too hot and turned off again, and repeat, and repeat.

If your boiler is start stopping not sure you will be seeing this. But if you are you still don't have enough of the heating system engaged to keep the boiler happy and cannot move the heat fast enough away from the boiler.

Does your boiler have an error code 202 when you have the UFH on? if it does you are in a protection mode, which is effectively short cycling?