Well, after some stop-and-go cutting with the plunge saw I've managed to
remove the suspect plank (and without flooding the kitchen):

http://bertcoules.co.uk/non%20websit...%20removed.jpg

(Short version: https://tinyurl.com/y9ytyasp )

My initial reaction was that I was surprised to see those particular UFH
housing-boards: I expected to find the ones for straight pipe runs,
which would surely give slightly more support. Of course I don't know
what's happening at either side of the exposed section: I suppose the pipes
might well be turning back on themselves there. You can see that the
pipework runs in two different directions and the two runs at the bottom of
the image do indeed turn through 180 degrees so the layout isn't as
straightforward as I would have thought.

The next thing I noticed - annoyingly - is that the plank I removed, the one
which dropped when weight was put on it - doesn't have a damaged edge. And
bafflingly, neither does the one next to it. Not only can I see no reason
why the depression was happening, I can't see *how* it was happening.

More investigation tomorrow...