When you say rockwool will still allow air flow to the roof space, wouldn't the inclusion of a vapour barrier exclude this? I'd be a little concerned that I'd get the Kingspan tight enough against the rafters or without voids when trying to infill 140mm; the rafters have been in place for 50+ years and some have a slights twist etc.
Could using something along the lines of Actis Triso multi foil help in anyway?
Yes use quilt between the joists, to ensure that no voids are left and its tight up against the deck, then then fit your Kingspan to the underside of the joists and tape all joints and then plasterboard.
Obviously the roof void wont be a vacuum and there will still be air within it, but by eliminating all voids, in particularly up against the deck, and then preventing any moist air from entering the roof, then you remove the possibility of condensation forming within the roof
How much insulation would you recommend? Having just re-measured the joist depth i've found it to be only 135mm. Does say 135mm of Knauf Crown 32/44 and another 50mm of Kingspan/Celotex sound enough or should I try a get more PIR between the joists?