Knauf dritherm 32 insulation. Vapour barrier!

Hi,
Hope everyone is safe!
I insulating my1930s front bedroom bay window.
It has a small flat roof void above, I planing fill this void with knauf dritherm 32 slabs. I am a bit confused about if I need a vapour barrier.
The ceiling is about 8 ft by 3ft.
I am fitting insulated plasterboard under bay windows and side reveals.
Many thanks.

 

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