OK...

A poser:

As I understand it, sarking is for windproofing, not water proofing a tiled
roof?

Then, if we install celotex between and under the rafters and therefore
become concerned that we maintain good ventilation above the celotex, then
why not just remove the sarking and let the roof breath through the tiling?
The celotex (at al) will give us our wind proofing...

Patches of my sarking are falling off and I see no evidence of water ingress
in those areas, but I do feel the air moving when the wind blows.

Just a thought...

Tim