Will that provide the U-value you need for compliance?I have had a look into this and I think can use Celotex 80mm insulation board.
You'll need a vapour control layer, e.g. foil-faced Celotex, foil-backed plasterboard.I will have to look into this a bit futher but I am lead to beleive that this will create a warm roof and will eliminate condensation problems.
No it will not provide the required U-value but I wont loose any sleep over this because its not like the house will burn down. I will also use 25mm Celotex (foil faced) under the plasterboard.