I have just ventured outside to check on the water barrels and both are frozen. The one has the expensive supposedly insulated cover which was bought for in excess of £30 from a place that specialises in making these covers!
Can any one make a suggestion how to defrost the barrels as we probably cannot bring them into the caravan?[/quotet
Can't help with the defrost. But can suggest you copy me. I have one of my water barrels fitted with a insulated cover, as you do in winter. But the fit is so bad, open hole round the top. Ideal for heat to escape - heat in both air and water rises. I have added a waterproof aquarium heater, it uses about 400 watts. And in twenty years, even -18C have never had a frozen water barrel.