IMAG0112

If you could fit a cavity closer available from someone like eurocell that would be good, measure the cavity width though as there are 3 different widths of closer available, if not I would just use expanding foam with a bit of dpc right behind the outer skin although foam is waterproof in itself, once dried trim it flat then dot n dab your new plasterboard over it

Hi, thanks for your advice, I did wonder about expanding foam, but how would you stop it from spilling out of the cavity and onto the floor?

It wouldn't, obviously start at the bottom and work upwards, they'd be enough grip on the bricks to stop it falling out