On 25/11/2017 17:43, 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.)

Bloody hell what sort of ceiling structure is this? Assuming it is
plaster, rather than concrete, I would probably look to stick in a
suitably shaped bit of plasterboard first, prior to filling with one
coat. Or perhaps fill with scrunched up chicken wire, then plaster of
paris. I'd still expect to need a "finish coat", the modern lightweight
stuff as other posters have said is very good.