And finally we have the answer. According to malwarebytes support, the latest Windows update caused this conflict between Chrome and Malwarebytes.... If only I'd known that hours ago... There's a setting buried that's enabled me to turn off Chrome as an app covered by Malwarebytes so have turned that off for now following guidance from support.
What an absolute palaver. Glad I didn't reset or buy a new laptop, as I would have just run into exactly the same problem and think I'd have lost it at that point....
Would have been really helpful if restore had actually rolled back the updates, but after it restored it restarted and installed them again before I could even get in to turn them off, so restore would seem to be useless for rolling back updates.
Anyhow there's a thread about this on the malwarebytes forum - 1000s of people affected, some who can't turn off exploit protection for Chrome.
Jeez, what an absolute palaver, or did I say that already 😲