I have 12mm plasterboard about 5-10cm out from a 50cm stone wall.
There is no vapour barrier.

Is there any sort of insulation that can be used in this sort of situation
that will not cause condensation problems?

Or is my next task to either replace /add foiled plasterboard, paper the
insides with some sort of vapour barrier, or invent a 2" deep robot that
can climb along the inside of the walls and spray bitumen.