You see, it would all be so much easier if (a) I remembered what I had already learned and (b) beekeeping was more science than art.
Max (apiary warden) and the bee inspector (David Rudland, I think) noted that the bees had particularly bad infestations of varroa. Part of the problem was that I had put the Apiguard trays on top of the crown board rather than on top of the brood box. This, I am told, makes it less effective. I know pefectly well that the Apiguard should go on the brood box. D'oh.
The second part of the problem was that some beekeepers think it makes no difference - it's purely whether the bees actually take their medicine, not whether it's on the crown board or brood box.
Ask two beekeepers, get three answers.