No ventilation in my soffits?

As per a recent thread, I've spent a happy(?) weekend ripping down the old lath and plaster from the slopey ceilings of one of our upstairs bedrooms (cross section of the room looks something like this: ________ / \ / \ / \ | | |____________|

As there's currently nothing between my daughter's bed and the sky except for one layer of lath and plaster and one layer of rooftiles, I'm fitting Kingspan between the rafters and overlaying the whole lot with insulated plasterboard with a view to reducing both the whingeing next winter, and my fuel bills.

Anyway - I'm mindful of the need to maintain an airgap between the Kingspan and the tiles to allow a through draught... but having exposed the rafters today I find there is no gap to the air at the bottom as I'd expected; it's totally sealed. There's a wallplate on top of the solid brick wall, and the rafters sit on that; clearly the house must have been built that way (1912). No sarking; tiles are bedded in original mortar which is crumbling somewhat.

I can't see any way I can introduce any ventilation in there; there's no eway through to the soffits. All I can think of is to fit ventilation tiles in between every rafter (that would be about 20); it would be awkward, expensive and look pretty hideous on a period property.

Is there any other way of doing this? Or should I just ignore it? Gut feeling says that although it's been fine for 100 years and all the timbers are fine, it might be a different story once I introduce

Any thoughts please?

Thanks David