When I visited last week there were dozens of bees flying in front of the wall. Most of them were female hairy-footed flower bees (Anthophora plumipes), with only a couple of males (I think most will already have died off after mating). The females were bringing pollen and nectar back to their nests and they searched for a few moments before finding the right hole and diving in.
There were also quite a few cuckoo bees, Melecta albifrons, a specific cleptoparasite of Anthophora plumipes. They were easier to photograph as they were settling on the wall and walking around from hole to hole trying to judge which nest to invade. Interestingly the incoming flower bees paid them no attention. Melecta bees have a different shape with a flatter abdomen and no scopa (pollen-collecting hairs) on the back leg. Most have white patches on the abdomen and legs although some are more or less completely black. They also have brownish wings and a much quieter buzz.
I also saw a few tawny mining bees (Andrena fulva) at the outer and lower edges of the wall but didn't manage any useful photos. I hope to find some more elsewhere soon.