SOLVED.

The problem was that when Office 2010 was first released, I just
downloaded the 60 day trial to get going quickly. The Trial version,
for some obscure reason, does not install add ins in the normal way.
It had not expired, so I had not got around to upgrading.

Charle's post prompted me to do that. And now it works. After about
a long day of stuffing about, hunting the registry & web etc. and
almost starting a substantial rewrite of the installation.

So naive users that use the trial with a custom add-in will have
difficulty installing it. It is very hard to see how this could be a
bug, nor why it would be on purpose.

I have often wondered about the other different editions of Excel.
Are they really the same Excel, or are they different builds with
different quirks?

Thanks for the help,

Anthony