Thanks chaps, this is really helpful.
It's a reasonably large 3 double bed house. So we've got rads in Lounge, Kitchen, Utility, Hall, Cloakroom, Study, Bedroom x 3, Landing and en-suite. So I make that 11 rads unless I'm mis-counting.
So is it safe to say 82000 BTU divided by 11 Rads...