On Dec 10, 2:09 pm, The Natural Philosopher
wrote:
Would I have to line the existing chimney up to the roof?
Surely it should not get very hot except close to the stove,
or the heat-loss would be very large?

Oh, it is. About 70% of the heat gets wasted..

from a good stove? nah - open fires lose 80-90 don't they?