The Polycell Base Coat is good stuff (we have a slightly flakey kitchen ceiling). However, bloody hard work to put it on as it is a bit like painting using plaster. Read that as your arms won't half ache afterwards
or use Flash and Cif.
we find that will shift it... I'm a DECORATOR by trade.
Once clean.. allow to totally dry.
what paint finish is on the ceiling at the mo?
Matt? Vinyl Silk or Gloss.. or Eggshell?
Clean down the ceiling of any flaky paint.
spot prime bare areas with thinned down emulsion.
use filler to fill cracks and to level off the 'mapping' of the flaky paint.
finish with two coats of soft sheen paint.
this will make it easier to clean down next time.Use a quality TRADE paint.. say Dulux..as you will use less coats of paint and it will cover better than Retail/DIY paint you buy
If the ceiling is badly 'stained' and they keep coming through... use PolyCell stain block (spray or paint) to block the stains out.