Peter Duke
Hi This is a result of damp walls , the efflorescence is salts coming through the plaster from the brickwork. To cure you will need to remove 1m of plaster along the affected wall , have both inside wall and outer wall treated with damp cream, then plaster with limelight and scim . Also inspect the pointing& gutters/ down spouts for leaks. Penetrating damp will be another cause . Treat external brickwork with good quality weather shield. Good luck. If you are in my area call me and I will come and inspect
