John Rumm wrote:

They are usually a fairly tight fit, so you may need to tap it into place
with a block and a hammer.

For fairly tight read impossible, and for tap it into place read shave a
(very small) bit off one long edge. Fixing the deflection deprived the
plank of its original full-width, non-flat-lying home, I think.

And now the job is done, everything is back in place and the floor doesn't
flex beneath my feet any more. Many thanks to everyone who chipped in with
ideas, thoughts and suggestions: all much appreciated.

There's still movement elsewhere in the area, caused I reckon by the
un-flatness of the underlying floorboards; I wouldn't be entirely surprised
if another t&g join were to fail some time in the future. But I'll be ready
for it next time...