This leaves an ugly hole about a foot deep and very wide.
What I want in the same location is an 8" wide hole 18" deep, to take
the new post and the concrete around it.

At the moment all I can see to do is fill the hole in, with much
tamping down, then dig the 8" hole in the middle of the newly filled
in larger shallower hole.

Does anyone have a better way to do this?


Are you worried deeply about the environment and landfill? If not, can
you source (eg from local builders or decorators) some empty 10 or
(better) 15 litre plastic paint pots? Cut bottom out and use as your
"mould". Back of Excel calcn suggests 15l is a little less than your
8x8x18 inches (which I make to be about 19 litres) but as others have
said you get added benefit from the mass being lower.

One other thought: make your mix weak if you think you (or anyone you
love) might want to remove your posts. I learnt that lesson from
uk.d-i-y only after putting in 10 concrete posts a while back. I now
hope desperately the posts outlast me as I stupidly used a strong mix
that goes rather deep.

--
R