I've just lifted the top cover and investigated. Quite surprisingly complex inside for what it is, seeing as it's essentially one giant heating element and a load of relays. Anyway - 12kW. As it should be near enough 100% efficient (bar the LED on the front cover!) then yeah, it'll be belting out 12kW of heat from the rads at full chat. The electric boiler does not heat the hot water - that's done via a standard 3kW immersion.
The trouble with this idea is that the radiators are big old 1970s (roughly) slabs that take up masses of wall space and probably don't chuck out much heat. So I was going to replace them all with new rads according to my heat loss calcs. However in the grand scheme of things, a few hundred quid on radiators is not too much to worry about.
Yes I was going to stick the thermostat in the hallway. There is a entrance hall affair to cushion the cold draft from the front, and we seem to be using the back door most of the time anyway.
I'm now thinking of going for the 24kW model. Bearing in mind the electric boiler and immersion combo together are 15kW and there are a couple of rooms (including the 3rd bedroom) with no radiator, I am beginning to think the 18kW choice is marginal.
Luke