Quote Originally Posted by wilfy View Post
i'm now thinking of using something like this

Polystyrene 25mmx1200x2.4m - Polystyrene Insulation - Insulation -Building Materials - Wickes

i'm still not sure if i need to shape something like that to the contours of the roof or not, preferably i'd sheet the whole roof in that, fill any gaps to the sides with expanding foam and line the joins with that foil tape, but i'm not sure if condensaton will still build up in the air gap or not?? could anyone possibly comment?
You didn't bother clicking the link I posted did you?

The stuff I linked is basically a thick pastic bubble wrap type stuff coated with shiny thermal film. It is cheaper per sqm than the stuff you are looking at and performs better as insulation.

If you read the comments, people have used this exact stuff for your exact situation and seem to be pleased with the result. ;)