Sorry to loose the thread - I was away for a few days.
Photographs below show the bees that have died and the comb that I think they were on last.
On the BBKA forum it has been suggested that they may have died from any of the following: the cold, acarine, nosema, glass quilt and damp. Maybe the photo's will help diagnosis?
Roger wrote: "It is difficult to give advice from a distance as it is much better to see the colony.
Personally I think you can almost forget nosema and acarine. You certainly didn't suffocate them either.
Did you have them the "warm" or "cold" way. I have seen several colonies die over the years when hit by cold weather and they die with plenty of food in other combs.
I think it could have been a small late swarm that couldn't maintain itself.
When did you hive it?
What varroa treatment did you give it and when?
Was the queen laying?
When did you last inspect it?"
...I treated all my hives with bayvarol in the autumn.
There was brood at this time.
They were cold way.
It was a late swarm hived in August just before taking to the moors and quite small - 3-4 frames. The queen was marked and must have been a 'domestic' swarm.
BTW The hive next to this one also appears to have died out. But it has a lot of wild comb and I'd need to dig all of this out to be able to have a proper look inside.
Thanks for any more help.