After much investigation we went for cork bits as the main insulation. These were sourced from Espadan Corks who run an fsc cork forest near Valencia. It's possible to get rigid cork batts too, which we ordered for the perimeter, but as these go to a central processing plant in Barcelona and get mixed with other growers cork they can't guarantee them fsc. They're also much more expensive.
We had to construct a frame to take the cork bits which involved a lot more work. We used the same wood planks we'd bought for the ceiling planks and put sheets of plywood on top.
Above: the frame and plywood topping
Above: the last coat of wood protection
When we eventually came to work with the cork it was a delight. It was warm and safe to touch, had a delicious smell and is naturally fireproof, bug proof and water resistant. Couple that with all the wildlife helped in the fsc forest and I think cork has to be an all round perfect insulator for this region.