What type of roof is it? Trussed rafters or traditional cut roof? Do you have pictures?

If trussed rafters then its main purpose is longitudinal bracing and loads are likely to be low, unless there are large cut-outs for chimneys, dormers, etc. If cut roof then its likely to be performing a load spreading or ridge beam function.

The more usual solution is to fit a ridge vent, so both sides can ventilate to the outside in a way that encourages stack ventilation. This can be done using a dry ridge ventilation system based on flexible corrugated rolls & mechanical fixing of the ridge tiles, instead of mortar. Is this an option?

http://www.klober.co.uk/products/fitting_instructions/Roof_Ventilation/uk.fitting_instructions.Roll-Fix_Kit.pdf

David

Here's the roof inside and outside:

But if you've got a 50mm gap, that implies non-breather felt, and it's the 50mm under the felt that needs ventilating, so leaky slates won't help. If it's breather felt, and the slates are really leaky, then you can fill the 50mm with insulation - no ventilation needed.

Wow wot a roof!! No trussed rafters there then

Pity WeeBeastie never told us how they sorted out the ventilation discussed here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13456&page=1#Item_22

Maybe the same solution would apply in Yorkshire.