Another vote here for Celotex. Not cheap but bloomin' good stuff..! I used it in the workshop between the wooden battens as people have described, plus I have three inches of polystyrene under the base.
It's better insulated than my house..!!
It's a b'stard to work with, though. Cutting it throws up a lot of dust which gets everywhere and isn't that nice to either breathe or blink in.
I drove up to Derbyshire to get a load of excess off a site for a deal, and keep an eye on eBay if money's tight.
