I've now done a reinstall / retaining. No better, 2 more fails already. Well, worse,, actually: the first time there was definitely no orange dot on the Restart or Shut down options, and now it does not even report an error -just fails and comes back after "Repair" as a normal boot - with the immediate attempt to download again if you don't very quickly stab the Pause Update button.
Having had the "bonus opportunities" to carefully watch the fail process, seems very clear that after normal "Configuring" power down, BIOS attempts restart, searches SSD, another HDD and the DVD for a bootable sector or partition, and fails. For 2 or 3 cycles you see the brief flash of activity, then nothing, just the blank screen.
So it seems very clear that there is some fragment of incompatibility between KB5016616 and the BIOS/hardware. On random systems with apparently no common factors whatever. Get on the case, Microsoft!
Oh, and it's not the AV software - my Norton was not even installed on the first occasion.
Mine is ASUS motherboard - any connection?
NB I have had a long telecon, and long chain of mail from supplier (PC Specialist). Full marks for support to 23-month old machine, but still no progress. I hope they are contacting MS.