Insulated plasterboard baton fixing

I would batten over insulation then secure board to battens. Will be cheaper and the insulation will protect the battens from any damp. If you are concerned about moisture put a vapour barrier on the wall prior to insulation. Use masonry screws straight through the battens and insulation into the brick rather than the mortar to get a good fixing. By a screw long enough to get minimum of 50mm into brick. Rich

Check to see if there is any penetrating/rising damp on the masonry. If so have that treated first. If not use treated (tanalized battens) Fix using frame fixings or plugs and screws into the masonry. Not the mortar joints. Level and plumb the battens up using packers if necessary. When setting the battens out have a look at the size of the boards you are going to use and set the battens out according to the details given on the product sheets. Many have inked marks on the finishing surface showing where to fix the centers of the battens for maximum effect. Fix the battens using proprietary board fixings at designated centers and space for maximum hold. There is two board usually. Tapered edge and square edge. Both can be skimmed using the board or multi finish after taping the joints alternatively the tapered edge boards can be finished using the tape filler and slurry method(Aimes Taping or dry lining process).

I would spray lightly over dusty brickwork and then apply Damp proof liquid to reduce any cold bridging, also reduce dusting. If you have the room form a 50 x 50in stud wall at least 25mms in front to create a void for air gap, then insulate the stud wall voids with 50mms insulation, then fix thermal board on the front of stud wall, then skim.

Ideally you should cover bricks with waterproof render then you use whatever method of fixing you prefer as you know there will be no damp issues in future providing rendering is done properly.