Thanks! that looks technically similar to the heat pumps aimed at space heating but a bit better - perhaps there's an efficiency in it being meant to do just one job? It looks like it (or its ilk) might be a good fit for me.

Outside wobbles around -2 to 8 in winter here, inside rarely less than 6, which is nippy but not dangerous. A not very insulated house with solid walls (this one's from about 1890 and has only had the easy things done) doesn't really sink to outside temperature if you live in it, cook, have appliances etc. There are plenty of heat inputs, just they do another job first. Hence my interest in water heat, I think it would start to be grim without it. Stove for a small number of really cold evenings, plus visitors. It would be a really bad idea to use it all the time but it would be hard to fit a radiator-type system to the job of "one hot room occasionally".