John Rumm wrote:

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

Yes, thanks, that's the first thing to try tomorrow, I think. Though a
slightly half-hearted bit of foot-pressure on the exposed UFH system (I'm
still a bit wary of damaging those pipes) didn't move it downwards at all,
so quite where the plank went to when it moved remains a bit of a mystery.
Perhaps I just need to be bolder.