Ewen,
You may be able to do a post-mortem on the colonies that (sadly) died. From the residue of bees on the hive floor or in a cluster on the frames, there may be bees with deformed wings which is a sure sign of varroa damage. You might also see deformed bees that are dead in their cells or where the bees have not been able to get out properly. (Some tweezers are useful for this). If there are bees that are dead in the cells but head-first, that indicates starvation. If there is food in the hive, then isolation starvation would be the problem where the colony has been unable to break cluster to get the food it needs to keep warm.
You may be able to do a post-mortem on the colonies that (sadly) died. From the residue of bees on the hive floor or in a cluster on the frames, there may be bees with deformed wings which is a sure sign of varroa damage. You might also see deformed bees that are dead in their cells or where the bees have not been able to get out properly. (Some tweezers are useful for this). If there are bees that are dead in the cells but head-first, that indicates starvation. If there is food in the hive, then isolation starvation would be the problem where the colony has been unable to break cluster to get the food it needs to keep warm.
May your bees read the same books as you do.