Thank you Roger. We are now weighing up going with an overlay board which is fairly low profile but of course not ideal, but better insulation. Are you going with JK ? - they look really good and have done some major projects inc museums and listed houses. I am very impressed with the cost too. Still torn. I know of someone local who had it done in concrete and is very pleased with it, so I think its certainly an option.
And just to say its oil Joth- we live in a rural area.