Well Crappers.......... the first thing I'm going to say is... do fit an extractor fan over the shower/bath ;o)))
Back to the plot .... scraping, then more scraping. Scrape everything that will lift off. If any is still solid, then it would be OK to leave it on........ hopefully ;o)
It'll have to be well sanded to try to get rid of the edges. Give it a couple of coats of diluted PVA, then repaint.
After you have scraped it off I would use a anti mould and mildew remover just to make sure it wasn't either of those that caused it to come away in the first place. Then wash it off as sometimes it could be something like ceiling white that caused the flaking. You will know if it's this as the paint will all come off on the cloth as you wash it. Then I would use a couple of coats of watered down emulsion as a sealing coat often called a mist coat then give it 2 coats of emulsion to finish it. I don't like using PVA as a sealer for paint as it has a habit of making the paint bubble as it dries.
Thank you both for your replies. I will probably do as shedman says as I haven't got a clue how to fit an extractor fan! I shall have to carry on leaving the window open in the bathroom when I'm having a shower.
I had a lot of problems in my bathroom regarding damp and flaky ceiling, spent a little bit extra and got the plastic cladding 10 years on and still amazes me how good it looks