Is it better to use a mortar or and adhesive to dot and dab plaster board to walls?David?

I am trying to fix plasterboard onto brickwork/cavity closer in window openings.

In a sense the question is a bit confusing. Are you installing DRYWALL to studs? OR are you boarding over and existing wall material? Is PLASTERBOARD the actual substance?

Have you defined the differences between Mortar and "adhesives"?

Certainly if this is material going over studs or existing wall material, one might use a contracting adhesive, but it wouldn't be my choice as the only method to secure new wall material.

I'm going to assume this is not Lathe/ Mesh/ and Plaster being installed.

Just my two "sense"

If the background you are fixing to is very rough then I would dot and dab with bonding compound. If the background is straight and true then I would use 2 by 1 or 2 by 1.5 battens. Then simply screw the plasterboard to the timber making sure the battens are centered according to the width of the sheet. Normally 1200 is the size of the slab so your battens will be 600 on center vertically with one batten running horizontally on top and one on the bottom that will also give you something to fix the skirting board to. If you decide to dot and dab, fixing skirting boards and corner beads will have to be stuck on by adhesive. Bonding compound can be used for corner beads as long as you sponge off any excess leaving room for the finish skim coat.

Use joint compound, the same stuff used to finish the drywall.