Thanks for all your replies. I've just returned from reversing the nadirs on the seven hives in my HQ apiary, prompted by Ian123's post above.
One colony has been virtually wiped out - all I found was a 2020 queen (purchased from Hivemaster in June) and a couple of hundred bees. I've put them in a small nuc but the wasps were entering again as I left. I should have acted much sooner. The colony was much stronger eight weeks ago when I treated for varroa. Presumably defensive traits of the workers are inherited from both parents.
Most of the nadired shallows were empty, having been fairly full six weeks ago. The stores have been moved up into the BB, also robbed I guess. Previous years I have left the shallows below the BB over winter, believing it shields the bees from draughts. I do not normally like moving boxes around at this time of year.
Lest you all think this is a tale of total incompetance I stress that this wasp invasion of most of my hives is something new . My nadiring practice hitherto has served me well. My two out apiaries (four hives) are doing well.