We have now insulated beneath all of the ground floor by removing the cellar ceiling and stuffing the gap between the joists with 200mm of hemp (except over the biomass boiler where we used 200mm of rockwool).
This was a lovely material – cuts with a bread knife, smells like harvest time, and no more freezing cold feet. By cutting it slightly large, it holds itself in place with no fixings. It was supplied by Back to Earth (see links for more info).
Eventually we’ll put the plasterboard back, which will be even better.