Insulate edwardian roof

Hi

I have just had a leaky valley gutter repaired and large timber rafter replaced (leak was several years old and timber rotten). House is C1900 and 2nd storey room in roof (Vertial walls about 1m high then pitched) Roof is slate (no felt or membrane) with lathe and plaster. As there is currently a large section of plaster missing though it would be a good time to rip it all down and to insulate. Main Rafters (down Vallleys and across mid height of pitch) are 10" deep, but rafters that are plastered onto are 3"(75mm)x2". Looked at Kingspan info and it states 50mm gap for between and under insulation. If I use 70mm TP10 kingspan would the 5mm Plus the gap provided by the cross battening that the slates sit on, be enough ventialtion to prevent condensation.

Also TP10 insluation (for between) and K18 (for underrafter) both seem to have foil, surely foil is only needed on one as a vapour barrier?

Any help welcome.

As not reply so far, Have looked into it a bit more

How about 25mm of K7 between rafters (leaving 50mm + counter battens) then K18 plaster board to maybe 25+12.5. I know its not up to regs, but its got to be better than a bit of 100year old black lime plaster.

Could I also slap some 2x2 batten on wall and fill with K7 and top with K18. All the diagrams should VCB, would this be needed in a solid wall that has survived happily without one since built.