Hi Ed, Billi,
both good suggestions, and things I've thought about and not ruled out. The YAGNI was my first plan but it really depends on how much power I can reliably produce renewably, if it's borderline then I may go down that route but if it's likely then I'd rather have a chimney designed into the fabric to give me some interior thermal mass to compensate for the bridge

also, by the time I've laid all the plumbing ready, sized the store and had a gravity coil fitted I'm only really saving the price of the stove itself.
The wood gassifier, external furnace or whatever I do like but my new shed is going to be quite a distance away, though there's nothing to stop me building a dedicated one, food for thought chaps, thanks.
Cheers, Paul