Thanks - just tried that, came to 16.53kw, incl 4kw for DHW as I have fast recovery cylinder.
I actually have 15 rads of various sizes, mostly K2, but only use 12. Separate 2-way valves for downstairs, upstairs and DHW, with room stats in hall and landing and TRVs on nearly all rads. Bathroom (with TRV) is on DHW circuit, which is nice. Run upstairs at 18C and downstairs at 19C, bit more when wussy relations staying.
Thanks everyone for a lot of useful stuff. Think I'll stay with open vented, but still have to decide whether 18 or 24kw.
Ref. the Range Rover or Mini, I chose second-hand 2.7 V6 car over 2.0 (diesel of course), but the subjective driving pleasure doesn't quite read across to a boiler!