Wilfy, I have just finished (well nearly, there'l still be bits to finish in 5 years time according to the fuhrer) building my house and have had many conversations with the building inspector about "warm roof construction. I think that is probably the best solution,but 50mm foam would be better. the problem is water vapour which will pass through almost anything, so needs to be completely sealed on the warm side. Any moisture the that does penetrate the internal vapour barrier will condense either in the insulation(interstitial) or on the underside of the roof sheet.

The sprayed foam insualtion would almost certainly provide both the insulation, vapour barrier, seal any voids and sticks like ----, however this is probably quite expensive.

Good luck, G