How thick is the plaster on the reveals? When you say you're studding out the walls, i'm assuming you're studding onto/over the old original plaster, rather than hacking all back to brick. If it was me doing it your way, the only plaster i WOULD remove carefuly, is the plaster on the reveals and soffit, take it back to brick/concrete lintel. By removing the plaster from the reveals etc, that has given you, hopefully enough depth to fix some plasterboard to the reveals/soffit when the time comes.
Right, here we go, this is what i'd do.
Measure and fix your studding to the main part of the wall, making sure you "start from the exposed brickwork of each reveal, and work away from them. Keep the edge of the battens plumb to the edge of the brickwork on each side of reveals, and the same with the batten along the lintel. Keep the batten horizontal and level/flush to the underside of the concrete lintel.
Fit the rest of the battens, insulation, plasterboard etc as per normal.
Once you have that all done, it's time for the reveals/soffits to be boarded. After removing the plaster from the soffit/reveals, you should now have enough depth for the plasterboard. The reveals and soffit will now also be wider too, because the wall has been battened and boarded out, so now, starting with the soffit, measure and cut a piece of plasterboard for the soffit, then fix it in place. You can use a grab adhesive, (sticks like st*t) or similar to bond the plasterboard in place, plus you can fix the plasterboard along the front by screwing into the batten that was fixed to the lintel. You can then do the same with the reveals on either side, (grab adhesive and screws into the timber studding) running down the side of the reveals. Bring the plasterboard on the reveals out flush to the corner of the wall/reveals. One you've done all that, it's just a matter of putting thin coat beads around the reveal/soffit, taping all the joints in the plasterboard, then you're ready for plastering. Hope you get my drift on all this. Remember, "fix the plasterboard to the reveals and soffit last", and good luck to ya.
