That must be worrying Murray. Puzzling too. I was thinking of chipping in to say that I saw two sealed drone cells in six colonies (!) on Saturday, but maybe the worker brood is of more interest.
So - six survivors, one loss, plus another really weak (2 frames) Varroa-damaged one which I expected to lose and did. Sealed worker brood in all, up to about 6 frames (not 6 full frames, but some are) with 9 seams of bees. The weakest one has 5 frames of bees and 3 frames of brood. This is at my main site not a million miles from one of yours. Of the survivors, four went to the heather and two stayed put, where they had syrup. All have had fondant* on since the oxalic treatment in December and all are in wooden Nationals.
The LA colonies near Longforgan are very weak but one at least had some sealed brood a fortnight ago when we used it for a display an art student is making. Didn't check the others for brood as it was too cold.
Hope that things start looking up for you.
Gavin
PS These bees get a small boost from snowdrops though there were only a couple of flying days this year despite their very long season. They have been piling in willow pollen recently, now some cherry plum and Scilla too.
* BFP bakers fondant. Either a half or a full 12.5kg box. The strongest has taken the lot.