Firstly thoroughly rub down the ceiling to a nice flat (except for the holes etc) finish.
PVA and then use a trade filler like easifill (readily available from B&Q).
Apply to the affected ares where plaster come out(don't use plaster as the reason the plaster came off was because it was too...
I am in Aberdeen and that price sounds right to me,I'd have lined the walls as well for that price personally so I guess Zampa's right with it being very slightly over,certainly nothing to worry about.
You're lucky to get someone to do it so quick if they can do it before xmas thats a miracle...
Ok,there could be a few reasons for this but I will guess you haven't sealed your wall properly.
I think this because although some papers as mentioned are prone to shrinkage this is easily overcome by proper preparation.
I only use thinned down PVA to seal walls,a 3-1 or even upto 5-1...