Visited Home & Hearth at the weekend and was shown a log burner design specifically for low heat demand passiv houses, the Woodfire Passiv from these guys:
http://www.firepowerheating.co.uk/hearth-and-homes-exhibition-2016
Seemed neat:
2kW room
9 kW water
direct air only
room sealed
However it failed the SWMBO 'must look right' test - sorry no photo as the show had a big sign at the front entrance saying no photos allowed.
That'll be me back to square one then.
PS. I'm in no way associated with Firepower heating but they have been jolly helpful and seem a friendly bunch.