Super-fast check of the hive ... took each super off in turn and looked for capped brood... and sure enough, there are emerging bees in the super immediately above the queen excluder. In other words, the queen must be above (rather than below) the QE.
As usual, I could not find the queen, despite plenty of looking (and four stings). On the basis that the queen is where the brood cells are, I moved the super with the brood below the QE.
(Being a WBC hive, I have been advised that a single brood box is probably not big enough, and it is better to have both a brood box and a super below the QE. This arrangement is known as a "brood and a half," a phrase I trot out to encourage the belief that I know what I am doing.)
In conclusion, I noticed that here were *lots* of bees, so added another super to the top. The hive now has a brood & super below the QE, plus three supers on top.
Now I come to write this up, I realise that there appear to be lots of bees because they are all in the supers and not in the brood box.
Just when you thought Incompetence was dwindling, it rears its confused head!