Thank you all for your kind comments and advice - feel a bit more chipper today, (maybe also because I took PolyHive's advice about "the wee dram"...)
I had another look in the hive yesterday, and took some photos as it was sunny.
I found the queen, she'd dropped out of the brood box onto the super below - dead, of course; at least I can stop searching for an imaginary swarm of bees now
- PolyHive, what do I do about the dead brood etc in the cells, will the Acetic Acid deal with them?
- Hivemaker, I have Fumidil-B but did not want to treat "routinely" so I did not use it. Wish I had...the next colony will have every treatment known to man thrown at them, believe me...Thymol crystals and Oxalic Acid still sitting forlornly in the cupboard
- johnfly, that's a really interesting post, I wondered if that had happened here until I found the queen
- Chris58, I think there would be much ruminating and oh-la-la's over my frozen frame at the SLAA, but not much as regards "positive outcome" - sorry, but this has been my experience of French beekeepers so far...
- Asian hornets are indeed here, but I haven't seen one since Sept.
Varroa count was nil up until a few weeks after feeding with sugar syrup, then I noticed a few bees with DWV and (I think) K-wings.
I treated with Varroa Gard, checking it every few days, and dusted with icing sugar each time the temperature was high enough.
The varroa drop as a result was considerable after this - >50 most days, I guess due to Varroa Gard, as I'd used icing sugar and not seen any varroa drop.
By 10th December there were only 5 varroa, but still dead bees, but with few signs of the DWV.
(I understand that it's normal to see dead bees at this time of year, so did not assume the worst).
The last time I saw live bees was 12th December - it was just about warm enough to dust with icing sugar.
So: those of you who think the colony was too small to get through, I think this must be the case, and I underestimated the colony size to begin with.
When I saw fewer dead bees/varroa, I really hoped they'd make it (not the varroa...) but as someone said on another post about DWV, "the prognosis is not good."
Anyway, I'll post the photos, and hope that perhaps the experience will help someone else.