John wrote:
Got up the steps to patch some lifted wallpaper on a ceiling and
found that it had lifted due the ceiling having dropped a bit. I am
suspecting that someone had put weight on the L & P at some time - or
the nibs of plaster have just failed over time.

I am wondering if there is a fix - such as generous squirts of No
More Nails from above and then propping the sagged ceiling back up to
the laths - or do I have to accept it is a lost cause and rip it all
down (ugh the though of the mess in the new bathroom!!) The
insulation is that blown in recycled newspaper stuff which would all
need moving.

I'm in the same boat as you so awaiting advice with eager eyes!