Sorry for the delay getting back to you. After running SURT I tried again just a couple of updates, on a whim/desperation - they succeeded after a long time. So I tried a few more, they succeeded also. I kept going, the more I did/fewer in the list of available updates the faster the process went. There were a few isolated update failures, I ignored them and kept checking other items in the next cycle. Eventually they all seemed to succeed, at least I'm down to just a few very recent outstanding updates, most if which I don't want (e.g. W10 nagware update).
So for me the issue is 'solved' in that it seems the problem is worked around / spontaneously fixed.
Thanks for your help and attention though!