The 'swarm' which I caught two days ago turned out to be from Hive H. They had absconded from the hive leaving nothing behind - no bees; no brood; no stores - only a few wasps.
I returned on Friday morning before dawn. The box with the bees inside was sealed, so I brought them to Hive H with the intention of combining it. I had two empty supers, newspaper and a queen excluder. I planned to put the newspaper over the brood box, then put the supers with the excluder between them. This would create a space beneath the excluder through which the bees could move. I would have more chance of finding the queen and removing her (I didn't think that I'd be able to save her).
I opened Hive H to find it entirely empty. It was clear that I had re-caught the absconding bees. I blocked the entrance completely and then put the bees back in. I added honey onto the crown board and put the lid back on. They were sealed in, which seemed the best plan in the short term.
Even before dawn there were wasps around the hive. I saw 4 at once at around 0530hrs. Clearly the site has a problem.
I moved the whole hive, with bees inside, to my home apiary. I was all finished by 0640hrs.
They're settling back into their hive in its' new position. I've seen evidence that they're cleaning up cells. There have been dead Varroa on the floor. I've even seen a few new wax platelets. I haven't seen any pollen yet. Maybe it's too soon for brood.
There are wasps here too but I didn't see any getting in.
EDIT 29/08/2017: After I moved the hive and colony to Headington I saw undertaker bees removing dead brood. I think that they did leave brood which I hadn't noticed. That brood would have died from neglect. When they were returned to the hive they would have removed the dead brood. I also saw dead Varroa on the removable base board. This suggests that the brood and Varroa died at the same time, in the cell.