Rod wrote:

I ended up using Screwfix' finest (i.e. cheapest)
gripfill-alike-non-solvent - squirted it into hole and kept slowly
squeezing as I withdrew the nozzle. Left it at least overnight - then
filled the remaining hole, largely caused by shrinkage, with ordinary
filler. And all is well.

Any clues how to get over OCD and autism?

:¬)

I can also give a very big recommendation to Polycel(?) "big gap filler"
not that squirty foam stuff but the stuff that comes pre-mixed in a
tub. It's very grey in colour but I sucessfully used it to re-create the
missing 6" square area in the corner of a plaster/lath ceiling by using
abit of polysterene as a backing (only flat substance I had to hand)
then screwing some screws in for a bit more "grab" factor, finally using
big-gap-filler to fill what was lath/plaster.

Tiny bit of sanding and a spot of paint and you'd never know it wasn't
original.

Definately one for those otherwise impossible jobs.
:¬)