Well, my laptop came back from the workshop a few days ago. The technician's notes said, and I quote:
What we've found
The issue you reported was caused by a faulty memory and an outdated device driver.
What we've done
We have replaced the part listed below and updated some device drivers. To help prevent issues in the future we have also reinstalled your operating system and updated your BIOS. We then ran system checks to ensure your issue has been fully resolved.
What we've replaced
I bit my tongue and didn't ask if they reinstalled Windows after they "updated some device drivers" because although that's how it reads., I figured it was probably an unnecessarily facetious question.
The result is however that it's all working nicely now. My carefully installed applications is are all gone of course and my carefully removed bundleware is all back, but it's working properly and I'm a lot happier than I was a couple of weeks ago when it all kicked off.
Thanks again to Axe0 for jumping in to help so quickly.
Apologies to anyone reading this hoping to find a useful solution to a similar problem.