Dear BillC45,
Many thanks for your helpful answer. It's much appreciated.
I'm pleased to report that this problem has seemingly been resolved without any further action on my part, i.e. today I fired up the PC and was able to both access Virgin mail both from my Virgin Media page and Outlook 2010. I'm mystified as to what's been going on over the last two or three days.
In any case, to answer your points:
1. Windows 10 Home, version 2004, OS Build 19041.746
2. Thanks for the reminder about Outlook support. Like many others I've been resisting, but lately seriously considering, upgrading to Office 365 as I agree an upgrade is probably overdue. Just didn't like the subscription-only model.
3. Firefox is up to date (release 84.02). Not cleared cache and cookies but will do so...
4. N/A
5. I normally access email direct through Outlook, and occasionally from my Virgin account (as you describe). Outlook was giving me the message: "Cannot start Microsoft Outlook. Out of memory or system resources. Close some windows or programs and try again." My Virgin email link was giving me a purple screen with the pulsating dots, then "retry".(as I recall)
For now I'm happy that things are working again but the above might be of use to somebody.
Thanks again