Here's our efforts:
Opened up the old Baxi coal fireplace that we proved produced zero heat output:

Under the direction of our HEATAS engineer I built a sub-chimney which overcame the myriad difficulties that were presented by the odd shaped fire-place, throat mantle:

Then a 12KW Firebelly FB2 (4KW to room, 8KW to back boiler) :

The hot water from the stove gets dumped into the Mc Donald Engineering 250litre heat store.

Final part of the operation will be to put 2x20 47mm tubes on our South facing roof.