On Sat, 25 Nov 2017 17:43:45 +0000
GB wrote:

I need to fill a pretty deep hole in the ceiling. With one fill if at
all possible. Ideally, just some filler that you bung up, and it
stays in place. Any suggestions for the best one for the job (or at
least a pretty good one), please?

It's about 30 cms long, 5-8 cms across, and probably 5 cms deep at
the deepest. (1 foot by 2-3 inches across and 2 inches deep.)

One-coat plaster, preceded with a spray of dilute PVA to stop the
existing structure from sucking the water out of the plaster. Don't try
to get it perfect when you apply it, just get it pretty level and then
go back in a bit to scrape and polish it once it firms up.
I always used to use Polyfilla type stuff for this sort of job, building
up in two or three layers, but I find the one-coat mostly works better,
quicker and cheaper (of course if you have most of a bag of sitting
around unused then this last point may not apply).