Flat roof space insulation - ventilation questions

I am about to put rigid insulation into the ceilings of a number of attic rooms, having taken down the plaster to get at it (no insulation in there previously). The shape of the roof is sloping sections at each side leading to a flat section across the centre.

The sloping sides have plenty of ventilation through the eaves, but the central flat section has little draft from the sloping parts (mostly blocked from it by rafters), and no vents through the roof felt. Apart from leaving a 50mm gap, should I put in roof vents in this section of flat roof, or would a vapour barrier be better/suitable?

Similar question for the ceiling above the windows that stick out from these rooms and have a small flat section above - no ventilation for them, what is best insulation approach?

Thanks in advance,

(BUMP) No-one have any thoughts? I have all the Celotex ready to go...

Sorry no idea myself. Was wondering about a similar situation. my bathroom is made of an extension, ground floor with flat roof. Have cut out a small section of plasterboard of the ceiling to reveal about 7" joist height upwards to what appears stirling board. Loads of breeze in there and a few dead spiders- a good sign re woodworm etc- breeze and ready route in for predators, a good thing.

Though I want to increase the insulation, as there appears to be none at all.

This issue is a real tough one to work out; offsetting benefits of insulation agains reduction in airflow.

If you come across any diagrams of what the standard (?) construction of a flat roof is, then i'd like to see it. Otherwise my diy route will involve reacting to whatever I find, as Ifind it!
good luck!