Hi folks,

My house is exactly 100 years old. About 3 years ago most of the plasterwork
was skimmed. The original plaster was left in place and a new, smooth coat
was applied on top. I moved in 2 years ago.

The problem is that the old plaster foundation on which the skim was applied
doesn't appear to be sound in some places. In some areas the base isn't
stuck to the bricks so when I to apply pressure with my hand, I can feel it
moving up to and away from the brick by a few mm. Where this is very bad I'm
having to arrange for all plaster to be hacked back to brickwork and new
plaster applied, because the plaster is cracking all over, but there's one
area where there's just one crack, running from the top corner of a door
frame diagonally up to the ceiling. I previously repaired it by scoring it
out, filling it and repainting, but after a year the crack has returned.

I really don't want to hack this entire wall back for one crack - can anyone
advise a solution that'll last?

TIA.