Thanks JimH.
The following, I hope, will be of use to others, as for other people to be affected, something has changed with either MC or Windows itself, in the last 24 hours, to cause this.
I am now ignoring MC26 (I have the MC26 licence) and just focussing on trying to get MC25 up and running again.
I re-configured Auto-Import and unchecked "Build Thumbnails" and "Analyse Audio"
This fixed the Crashes (So far at least) and auto-import worked correctly
I then re-enabled Analyse Audio and again all was well.
I then re-enabled Build Thumbnails and the crashes recurred
I followed your suggestion and set Thumbnail creation threading to "Low" and then manually clicked build missing thumbnails and it went through over 44000 files successfully.
My concern is what has caused this change?
Is it a coincidence that I installed MC26 and this then prevents MC25 from working even on reinstalling it?
Did the MC26 install make any changes to Windows Defender exclusions?
I am just looking at the Windows Defender thread, but why would this only become relevant to me two days ago, which is when I installed MC26
(Coincidental possibly, but on the day I installed MC26, the only other change to my PC was a Microsoft Windows update that was installed on 13th Dec "December 10, 2019—KB4530684 (OS Builds 18362.535 and 18363.535)")