Thank you so much for all the feedback, Kevin - much appreciated.
I have stopped the all-in-one-go update from executing, and now I am
executing a series of statements, each constrained to update no more
than 100,000 records at a time.
Interesting fact: updating 100,000 rows takes 5 seconds. Quick.
I tried updating 1 million rows in one go, and the statement was still
running after 25 minutes, before I killed it!
So I am now executing an SQL script with almost 800 separate
update-statements, each set to update 100K records, and the thing is
trundling along nicely. (I am fortunate enough to be surrounded by
MatLab users, one of whom generated the 800-statement file in one minute
flat!)
Many thanks again for all the info.
Kind regards
Harry Mantheakis
London, UK