Does anyone see any fault with this plan, and do I need to install any
additional vents (if so, how - I really don't fancy climbing on the roof)?

If there is plenty of soffit ventilation, I think you may just get away with
it, although I suspect some BCOs might want to see ridge vents.

Shame you've got sarking, though! Without it, you could fully fill the
rafters and just put in breathable membrane next time the tiles/slates need
changing.

Christian.