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?