No Paul we haven't made a J tube stove yet but at the moment we're rushing to get the roof on to the extension before the end of October when the weather changes. We're gathering materials to do some experiments and reading up a lot on dimensions and designs ready for building the stove in the extension.
The building highly insulated 30cms of stone, 2Ocms brick with a straw bale interior. Do you think a rocket mass will be capable of heating the downstairs room plus the the two rooms upstairs ? I could also put a smaller stove under the stairs on the first floor if I need to as it's right next to the chimney but I suppose we need to live through a winter before we'll know for sure.