Help with ceiling insulation

We live in a very old cottage in Scotland.  There was little or no insulation. 

I need to insulate between the ceiling rafters in the bedroom.  There are also furry guests scuttling about that I need to evict.  I can’t access the space from the first floor above because the floorboards are very securely fixed and partially extend beyond my reach.

I need to pull down (some?) of the plasterboard ceiling to insulate.  Do I need to pull the whole ceiling down to fit a Waterproof membrane?  This old cottage is well ventilated (!) & we have no central heating.

Any help would be appreciated

John