On Sun, 14 Nov 2010 17:59:01 +0000, John Rumm
wrote:

You can almost ignore the bricks - they are of negligible effect
compared to the insulation!

Perhaps that's the easy way to get a rough figure. Work out how much
heat is lost through 50mm celotex and use that for my calculation. I
guess I also need to work out heat lost through the concrete floor
though?