I received some useful comments from my bee group about swarms: the first swarm to emerge from a colony is the prime; subsequent swarms are casts. A cast swarm will have a virgin Queen so it will take longer to build up because the Queen has to mate.
I had another look into Hive D to see whether there was a Queen. I didn't see one but I might have missed her. I looked carefully for young but saw only honey and stored pollen.
My bee group have suggested that the swarm might have left their parent colony with a virgin Queen. In that case it could take time for her to mate and produce brood.
I had a quick look into hives B, C and D yesterday afternoon. The news is mixed.
The best news is that I see no evidence of AFB. I'm not experienced enough to be certain so I'll continue to keep a lookout. I'll also review the NBU Foulbrood Diseases advisory leaflet.