After deciding to inspect the bowing in the ceiling, I discovered there had been a slow leak going on from the roof for some time. I carefully peeled back the wallpaper and a poke with the handle of a brush brought down the water logged plaster.
Now for a close up!
The original wattle and daub.
So I am now going to attempt to mend the ceiling using the original method. Wish me luck and any advice will be gratefully received :-)