It seems to me that it isn't really adhering firmly to the brickwork - although untouched areas seem solid enough, and sound right when you tap them, in areas where I have been chiselling out holes for backboxes it comes away very easily, loosened by the vibration, and once you start removing the darn stuff you can go on forever, as it all then comes away from the brick very easily. Perhaps the walls weren't wet enough in the first place when it was plastered.
Having chewed it over, and lacking any other answers, I will try working a little bonding in and then scratch coat and topcoat a few trial areas and see what happens. I just thought that if this was a common problem there might be a high- tech answer I didn't know about - soaking it with PVA or something, I don't know. Plastering isn't my game!
Thanks
JohnD
Have just read'plastering on painted walls' which seems to fit my situation as well. If all else fails I'll dot and daub. Sob, again.