Vapour barrier or no vapour barrier?

We spent quite a while considering this. Initially it seemed obvious that we would have one as most things you read have one, in which case our roof would be built up like this

The Outside

Slates

Batons (horizontal slats the slates are attached to)

Rafters (vertical)

Insulation

Vapour barrier

Ossature Metalique (literally metal skeleton - the metal framework that holds the insulation up and has the plasterboard screwed to it)

Plasterboard

The Inside

On a lot of roofs there would also be a felt layer between the batons and the rafters.

Intensive reading suggested that the vapour barrier like this is actually being used more as an air barrier than a vapour barrier, and that if you want to keep moisture out of the roof then the air barrier is more important. For example read this

Our roof outside the insulation is now very well ventilated, so we concluded that if we install the plasterboard well, and make sure we seal round all edges and get it airtight we don't need the vapour barrier - saving time, effort and money. What we need to avoid is water condensing out in the roof as warm moist air leaks out and cools and water condenses out of it.

We consulted with the roofer - he agreed we didn't need the vapour barrier and said he did his roof the same as we are planning. Which is lucky considering once we attach the ends to the suspenders they don't come off again, so you can't add the vapour barrier easily.