Boarding missing section in wall and boarding lathe and plaster ceilings with cornice

I suggest the Cheapest option that would be to just cut along the cornice and square off patch in the ceiling , board and plaster back in. If done by the right person you’d never know it was there

Q) 'I removed an old cupboard to reveal the bare brick. Will we just add wooden batons to the brick to fix the plasterboard level with the rest of the wall?' You could also as a cheaper option, dot & dab the plaster board to the masonry wall, you'd need a spirit level to check if everything is plumb/level. British Gypsum Bonding is what you'd use to dot & dab. Q) 'Removing the cupboard has also revealed a hole in the ceiling. The ceiling is lathe and plaster so we plan to board and skim the whole thing including the missing patch. The ceiling has, in part, some cornice. Does this need to be chiselled off before boarding? This is what one plasterer suggested.' Had a similar job to this a few weeks ago. Simply cut out a square of plasterboard, square that to the ceiling hole and pencil around it. Cut around that square, Use some spare plasterboard as backing studs to screw the patch plasterboard in place. Scrim tape around the patch and proceed to skim.

I would use bonding on both areas and skim. You shouldn't have to take the coving down, plaster up to it

If the cornice is a period feature, I would overboard the ceiling leaving a 10mm gap around the edge, and fit shadow gap stop beads which slot on the board edge to give a crisp finish