Nigel, as I understand it the water content of honey is determined by the relative humidity of the air it is in contact with. Data in Food Safety Management: A Practical Guide for the Food Industry by Yasmine Motarjemi & Huub Lelieveld, shows the relationship which is not a straight line. Basically to get honey down to 18% water relative humidity needs to be less than 60% RH. Your honey at 22% needed a RH below about 65%. Maybe there was so much moisture from the rape nectar the bees could not keep the humidity down.
I don't know what sort of humidities are typically found in supers in UK but the broodbox is around 50 to 60 % according to Arnia https://www.arnia.co.uk/hive-humidity/. If that air cools in the supers then RH will rise making ripening difficult. The bees must presumably re-heat that air in the supers to lower the relative humidity, which suggests any help from insulation would be useful. It may be analagous to the winter where bees in uninsulated hives can keep warm but just need to respire more to do so compared to bees in insulated hives.
I don't know what sort of humidities are typically found in supers in UK but the broodbox is around 50 to 60 % according to Arnia https://www.arnia.co.uk/hive-humidity/. If that air cools in the supers then RH will rise making ripening difficult. The bees must presumably re-heat that air in the supers to lower the relative humidity, which suggests any help from insulation would be useful. It may be analagous to the winter where bees in uninsulated hives can keep warm but just need to respire more to do so compared to bees in insulated hives.