1. Sand down ceiling to bare plaster if nec. to remove all loose/flaking paint.
2. Mist-coat any bare plaster patches you have created.
3. Apply two coats of Zinsser Gardz to the entire ceiling. (Expensive but worth it - it's an alkili resistant primer and combats damp in plaster).
4. Then paint whole ceiling with a good quality eggshell finish paint (tougher than ordrinary emulsion, wipe-cleanable and cheaper than so-called bathroom paint).
However...if condensation is a BIG problem in that room, there is no paint on this earth (other than Weathershield exterior gloss perhaps) which will not flake off eventually if exposed to endless steam.
Solutions (in order of how radical) :
1.Leave bathroom door open when vacating the room
2. Leave bathroom window open whenever possible for as long as possible
3. Get a much bigger better extractor fan and have it linked to the light switch. It should continue to operate for at least 15 mins after bathroom is vacated.
4. Install PVC tongue and grooved panels on ceiling - sound naff but actually look like wood and are condensation proof.This sort of product (others are available)
http://www.pvc-cladding.co.uk/products.php?cat=Pvc+Ceiling+Cladding&gclid=CP_9wdih5q4CFVEjfAoddGZIxA
5. Tile the entire bathroom including the ceiling, effectively making it a wet room