That guy didn't call back yesterday so I phoned up this morning but she was working on the premise that it was computer specific and not OS specific.
Anyway, after she'd run a dism /scanhealth, /checkhealth and a /restorehealth - all of which came back clean, she then suggested a repair install.
I told her I'd already done one yesterday but she proceeded to download the ISO.
I then made my point that it wasn't machine specific with others getting the same problem and that it should be elevated and let them try it, as it had been proven to be as a consequence of January's update.
Given all I'd done and she had done without resolution, she chucked the towel in and agreed to elevate it.
Getting a call back tomorrow afternoon.
I asked her to try a restore point on her own Win 10 machine and she'd get the same error as everyone else has.
In the meantime I've restored the machine with an external image - reinstalled Norton Security and now waiting for a new system image to complete - it always takes hours on that machine.
I'm then going to boot up into the advanced boot options to select Command Prompt and enter the cmds as in Fix #5 of this article, substituting UserDomain for the full name of my laptop and UserName for my admin name.
http://borncity.com/win/2017/02/22/windows10-version-1607-system-restore-error-0x80070091/You can get the full name of the machine by right clicking on This PC and selecting Properties.
I don't know when I'll be able to test them as there's still about a 3rd left to go on the image progress bar.
If you want to give them a go, to find which partition Windows sees your volume in before you enter the cd cmd, enter -
bcdedit |find "osdevice" which will give the partition letter to use in the cd cmd.
Just to clarify, that's a Pipe symbol before find and is the uppercase of \