Recently refitted and decorated bathroom - cracked/flaking paint

If the ceiling was a newly plastered ceiling, then once fully dry this should be mist coated with a thinned out white contract emulsion ( this penetrates into the new plaster thus giving you a perfect base for your further 2 coats of Zinnser Perma white Satin). This is a high quality 2 coat finish which will withstand high moisture/condensation and anti mould properties for at least 5 years. So more than likely your issue with the ceiling is bad preparation and poor quality paint.

It could be the paint mite not been sized the way is to sand it put up lining paper then paint it then it will be fine then.

Would have to look at the bathroom to be sure, could be a few things. Insufficient ventilation, does the bathroom have a vent?, is it a powered vent?, does it come on with the light or shower and ensure majority of moisture is vented out? Type of paint and quality, ideally you want a silk or a similar waterproof finish. Damp from another source, the plaster getting wet through from outside might be the cause, any moisture behind paint layer will lift the paint off the plaster and cause flaking.

If it was freshly plastered, was the plaster allowed at least 4 days to dry at room temperature prior to painting. Secondly, was a mist coat applied prior to the two top coats? Thirdly, was the paint used a bathroom suitable that can withstand high moisture areas?

Bathroom room's is very sensitive area. It's necessary to use high quality paint special for bathroom. Also if you have bad ventilation system in 2 months everything is possible. The idea is don't save money with cheap paint especially in bathroom or another sensitive area's