I own a 1979 Lockwood house with a longrun-type steel roof which is showing signs of distress, so I am considering a re-roof.

I imagine that the roof construction consists of battens running horizontally across the top of the solid timber Lockwood plank ceilings, with the steel secured directly onto these battens. Maybe there is a plastic (or tarred-paper?) membrane in there somewhere? What size would the battens be - 50x25mm / 2x1"?
I'm almost certain there is no specific insulation, although of course the ~75mm thick planking has significant insulation value.

Any ideas on where to get some information on the way Lockwood house roofs were built in the late 1970s?

If I have to re-roof - which seems likely - then maybe it would be possible even desirable to install some insulation after the old steel comes off and before the new steel goes on.
Ideas that I have thought of include

• adding a relatively thin layer - 20mm or so - of rigid foam, or
• doubling up the battens with either another 50x25mm / 2x1" giving an extra 25mm gap or a 50x100mm / 2x4" giving an extra 100mm gap. This would allow thicker rigid foam or maybe fibreglass wool [incl Pink Batts(tm)]

Has anyone done this or heard of anyone doing this kind of crazy thing?
Where would I get advice on this?