How difficult to paint a ceiling?

Glad

Does the decorator who gave you the quote know that it is onto fresh plaster? Or have you just asked for a quote to ‘paint a white ceiling’

the_lunatic_is_in_my_head

Don't think anybody has said such in the thread yet, use this paint:

https://www.diy.com/departments/tikkurila-anti-reflex-2-flat-matt-high-opacity-ceiling-paint-zero-flash-10-litre/6408070068618_BQ.prd

Putting paint on a ceiling is easy, putting it on evenly is difficult, typically will look find with the lights on but during a sunny day the sunlight will highlight any unevenness in the application. 

Paint above reflects only 2% of light resulting in a better looking finish. 

There are probably places cheaper than B&Q but £55-£65 is about the going rate and it's worth the money.