Hi Sophie,
I have got a house in London UK built into a slope so it is partly underground downstairs, which is similar to your situation with the thick stone walls.
I have got a Morso Badger secondary burning convector stove with a baffle, that burns off its own smoke, in the middle of the house and at one end of the lounge. It generates hot air currents. the metal chimney is insulated and no soot builds up inside it, and it saved me having to build a masonry chimney. The chimney heats the bedroom above, that it goes through.
It gives out the equivalent of two electric fan heaters or 6 units of something=btw's? whatever the standard measurement is. It has been in 2 or 3 years and is just enough in a very harsh winter for an insulated house. As I am relying on the chimney itself to heat the inside of the house and a rocket mass heater keeps the gases inside the house a long time before they go outside, it probably would be enough on its own. I want one as well it looks loads better than what I have now. The expert , Ernie said he was burning an eighth as much wood as before, but he may not have had the secondary burning type of stove with the baffle to re-ignite the smoke particles.
It would work like a storage heater keeping the heat in its mass and releasing it slowly over a long period of time, that is the man advantage, allowing the heat to percolate into all the small spaces around the house. I have heard that masonry chimneys do that too.
