Without seeing them, it is very hard to guess what was the cause.
It might help to take a sample of the bees and send them off to the bee unit at York for a diagnosis?
Also to be on the safe side, burn all the frames and scorch in inside of the hive with a blow torch.
Although like you I treated with apiguard, I also treated with oxalic acid in late December, it really does help. I then feed with fondant as the oxalic gives them a bit of a kick-start so they need a bit of food.