John Rumm wrote:

You might only get deflection of the tongue when there is force
applied...

In fact I wasn't testing the right area. The board that shifted wasn't
damaged at all: it's the adjacent board. Though its tongue appeared intact,
one length of it is completely separated: it's held in place by the
still-attached tongue either side of it which is why I didn't immediately
notice it.

So a second plank has to come up, I think - a rather less daunting process
now I've dome a previous one.

Chances are there is a dip either visible at the top of the UFH panels, or
under them such that they can push down a bit when loaded.

Another discovery this morning: there's no deflection at all in the UFH
panel but one area of the flooring isn't laying flat against it. With the
t&g intact it wasn't particularly noticeable but once one plank was freed to
move independently downwards it became immediately apparent.

I wonder if laminate laid over UFH needs more expansion room than the norm?
If that's the case, and if the fitters didn't leave enough space against the
walls, then the problem becomes a bigger one, given that the L-shaped
kitchen units are masking all the edges.