Thanks to everyone for their input...... much appreciated.

I'm 99.9% sure that I don't have a problem with damp permeating from
outside. It is all condensation forming on the cold brick walls. I think
I am going to go ahead with using the insulated plasterboard. My only
concern now is getting the best adhesive to bond the PIR foam to the
brickwork. Given that the brickwork isn't dead even, I would like to use
some sort of fill adhesive. Any suggestions for a grab/fill adhesive
that is compatible with the PIR foam backing board?

Thanks.

--
Paul Giverin