I would suggest that the winters here are close to as mild as yours. Only 2 days of frost last winter. The oceanic, temperate maritime climate ensures that the mean winter temperature rarely gets below 4 deg C.
Yes, 3' thick stone walls, which I've virtually draughtproofed.
Heating will be via a 9 kW multi fuel stove which will be burning peat. The license for cutting peat is £7 per annum, so apart from a bit of hard graft in the summer, heating it will be virtually free. The dehumidifier I'm putting in there will cost more to run in electricity charges.
Thanks for all the advice and I really have carefully considered it all, but my time window is very small to take advantage of a second pair of hands. Lou couldn't possibly help me with fitting the roof plasterboard/insulation and it is almost impossible for one person to do it. Unfortunately, to get more insulation at this point would take at least a week, which I don't have.
As I said earlier, I'm more concerned about the acoustical properties of the walls than the thermal properties.