They all died of starvation
Unfortunately, last year seems to have been such a good one for swarms that there weren't enough bees left to keep the colony warm enough through winter. I'd have united them with another colony but I only had one.
The colony built up very fast in the early season due to the strong nectar flow and once they decide to swarm it's near impossible to stop them from going and the colony was depleted to such an extent that there weren't enough bees left to stay warm through the winter.
The winter cluster couldn't keep the inside of the hive warm enough for them to break off and just travel to the next frame which was full of stores so they starved (characterised by them dead with their heads and bodies in honey cells).
Hefting the hive didn't help because they had enough stores to survive by, but with there being too few bees to allow them to move around inside more freely meant that they only kept to two or 3 frames.
Next year I'll know to look out for there being fewer wax cappings on the removable varroa floor and any that there are being on one side of the hive and not the other which could indicate that they aren't uniformly spread throughout the hive.
Ever the optimist; at least I still have the equipment and 2 full seasons of experience, and some other beekeeping friends of mine are very likely to need to split their hive as part of their own swarm control this year so I'll have more again before the spring is out, it's just disappointing that I couldn't see this lot through their second winter.