It's far more important to know something about the level of insulation in all aspects of the property, Fred, as you will know from your physics O-level that heat in a well-insulated device like a thermos doesn't escape very quickly.
I'm talking walls (cavity insulated?), roofs (lots of Celotex?), windows and doors (DG installed?), and to a lesser-extent floors (assume zilch unless built in the last 30 years).
Boilers come in big jumps between size models. If it is detached but reasonably-well insulated, I would guess around a 30kW output model. Better to oversize a bit in case you extend the house later. You heating technician will be able to assess this on site.