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.
My out-apiary is under sustained attack from wasps.
There are dozens around the apiary. They concentrate their attention on the weakest hives. This has turned out to be hives F and H.
I caught a swarm today. It was small - perhaps about the size of an orange once it had clustered. It was stuck to the side of the building where my out-apiary is. I suppose that it came from one of my hives.
I boxed the swarm this afternoon and then moved it to the roof this evening.
The swarm is surely too small to survive the winter so I'm considering adding it to hive H, which is has too few bees.
EDIT 12/08/2017 - this colony of bees had absconded from Hive H.