Just to update, the guys are here and I've pointed out the really unlevel areas.
Turns out, there is a massive sag from the centre of the ceiling that is out at the edges by upto 1½"!!
We've talked it through and they are going to batton out the edges and use the lowest point of the centre joist and reboard and skim on Monday.
They are now checking everything for level and it is an old house, so door frames are a bit out, which is to be expected but I would just always notice the light curving around the ceiling as it is that bad!
The guys said they can't see how lime plaster could have held up enough to have leveled it previously but it did have animal hair in and was well keyed onto latt.