It all makes sense now.
The seasonal bee inspector paid us a visit today and shed some light on my little problem with the WBC hive. Turns out I have a drone laying queen which means she did not get to mate so lays unfertilised eggs. The workers tear open the cells when they realise that the grubs are males and not females. This will lead to a failed colony.
Fortunately for me I have a nuc with a healthy laying queen at the ready so I now need to unite the two colonies. Not sure how I am going to do that since I don’t have enough lifts to put onto the WBC to accommodate a WBC brood box. Will figure it out a bit later.
The drone laying queen was the queen I found in the supers after our holiday and I then placed downstairs. It seems to be that I must have had 2 queens in the colony at the same time and the virgin killed her mum and was then trapped upstairs. All a bit strange but a worthwhile experience.