big snip

Two caveats to bear in mind. One is that to be sure that the water
heat output is what it says it is - ie you are talking about 12 kW; is
that total in which case a significant proportion goes to the room. Or
is this the water heat in which case just be aware of manufacturer's
specmanship as again if 12kW is going to water then there's an enormous
amount going to the room. Consider the surface area of the stove and
the surface area of the heating tank in the stove.

Secondly jusr remember that electricity isn't permanant and you may
loose your CH pump. Possibly worth while considering a back up petrol
gene. of a kW or so, so that you can have the house warm and some basic
lighting, and not have to worry about the system overheating.

Another option by the way is to use a Dunsley Neutraliser

http://www.dunsleyheat.co.uk/linkupsys.htm

which allows mixed heating systems without going to the heatstore
solution - I have one for a log stove / oil burner set up and it works
well with suitable thermostats to control the system.

Rob