Not that I'm saying it's the best method, but I cut out a square of plasterboard ( with a stanley knife ) at the two attachment locations and screwed a square block of wood into each hole ( into plugs set into holes I drilled in the brick ). As long as the wood is at the same level as the plasterboard surface, you can now screw the unit into the wood blocks (having taken care to leave a sufficient area of wood to screw into, taking into account the screws holding the wood blocks to the wall).
Andy.