Man at B&Q coughed up some electrons that declared:

You *still* need to arrange for ventilation of the void between the
insulation and the sarking.

MBQ

Already have a plan for that - soffit vents at the lower end and dormer
fascia vents at the top (no need for ridge vents as dormer roof is within a
foot of the ridge). Need to ensure that all voids are interconnected for
free air flow, but BCO is happy with solution, although a bit
unconventional.

50mm gap under the sarking above celotex at all points.

Cheers

Tim