GB wrote:

I need to fill a pretty deep hole in the ceiling. With one fill if at
all possible.

https://www.toolstation.com/shop/One+Strike+Filler/p20822

When you pick up the tub, you'll swear it's full of Helium. lightly
dampen with a spray bottle before filling, I find it doesn't shrink when
drying even when applied in a single deep filling, though you can "tamp"
it in afterwards and then re-fill with a harder filler if it needs to
withstand knocks, though presumably on a ceiling that's not very relevant.