+ chuck isolated, so I wondered if anyone could please suggest a trick
or two which might free the chuck in situ?

Success - there were many clever suggestions and the first one I tried,
which worked for me, was a decent basting in WD40 followed by clamping a
bolt in the table, lowering and then gripping the chuck onto it, and
giving the chuck a very thorough clouting with a rubber mallet.

I'm now waiting for a kelyess replacement (the original chuck is intact)
and the B16 taper is still in good (rust free) condition.

Thanks All!

DDS