David Pearson wrote:
So, if anyone can help me out with some advice on
cheap, but still good enough, insulation, I will be very
happy.

If you're going to insulate using Celotex, try contacting
http://www.secondsandco.co.uk/ for a price (I think they just do
Kingspan, but hey). It's what it says on the tin - seconds - but is
perfectly OK and a lot cheaper than your local builder's merchant.

David