Thanks for getting back to me,
I can get a 1.2x2.4m, 100mm celotex board for £32 or this Tri-ISO-Super stuff is £165 for 1.6x10metres.
This other stuff claims to be equalivalent to 210mm of traditional insulation but is only 13mm. It just staples on...
My room is 4.2 metres square and...