Regarding inverted roofs, I don't have any personal knowledge. I just suggested it as a possibility that ought not to be forgotten. But I do think it's a real alternative, not a crock of s**t. I understand from http://www.soci.org/Chemistry-and-Industry/CnI-Data/2010/6/Better-buildings that the first Passivhaus in England has one and I believe the architect is on this forum and could confirm it or not. If so, it's not too shabby.

"the system also includes a vapour-permeable separation layer to reduce rainwater cooling of the membrane"

That also reminded me to check the Passivhaus details book. Detail DAI 05 looks applicable to your situation wookey. Neither it nor DAh 01, which are the two timber-based flat roof details in the book, are ventilated.

You can borrow mine

or which bit of the detail do you need?

Just in case anyone reading this does find themselves with a form of a type 'a' roof that is bitumous layer under tiles and over battens/beams/blocks etc but no place for a vapour gap and wants to put in insulation, towards the end of page 2 of the discussion linked to below is an explanation of a simple and pragmatic approach to adding insulation whilst minimising the risk of condensation.

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=2924&page=2#Item_7

Perhaps I can re-open this thread with a question......
I am currently refurbing an old stone walled cottage in northern scotland and debating what to insulate the roof with. Local "wisdom" advises me that I need to leave the 50mm air gap between the sarking boards and insulation. However this only gives me a maximum of 100mm insulation between rafters and as there is no provision to ventilate the ridge I can't really see what the point of it is. As the roof is effectively a breathing roof is there any reason why I cannot insulate to the full depth of the rafters and leave no air gap. In Scotland the slates are nailed directly to the sarking boards and no timber battons are used. My roof has a breather membrane applied over the sarking on one side and nothing on the other side just naked sarking and then slates.

My thoughts are to full depth fill between the rafters with either hemp and lime or with hemp bats giving me a solid breathing roof construction. Is there any reason why this would not work? I like the idea of backfilling with the hemp and lime behind t+g boards which would then act as my finished cieling in the rooms. The hemp lime would also help to preserve the timbers I think.

Peoples thoughts greatly appreciated.