OK, stupid time.

Accidently bought 6 sheets of ordinary 9.5mm PB in 3x6' sheets. Need to use
vapour barrier PB for the ceilings on some bay window ceilings which will
be insulated with celotex above. I don't even think the use of vapour
barrier PB is that critical as there's a huge void over half the bay
ceiling as the pitched roof meets the lead flat roof of the bay, but vapour
barrier was suggested.

Any harm in using normal PB together with a vapour barrier or either plastic
or perhaps even bacofoil (turkey sized) stuck on the back of the board.

Seems a pointless waste to dump the board as I can't use it elsewhere.

Cheers

Tim