The roof is nearing completion now. For a "belts and braces" approach, we've installed vents at eaves and ridge level, to ensure a good through-flow of air above the insulation envelope and thus reduce the risk of condensation/timber decay. The ridge vents proved difficult to source, being a special order item from the usual merchants - with lead-time and cost accordingly. In the end I found one in a salvage yard, and the builders offered to make others to match, by cutting up a spare ridge tile. The result is not great aesthetically speaking, but I'm sure will function OK.

Here's a picture of the reclaimed ridge vent, and the replica finial in position:

And this is the builders' copy:

Unfortunately, whilst working on the roof, a few slates have got damaged; obviously we need to ensure these get repaired before the scaffolding gets taken down.