Thank you again David for taking the time to get back to me on this.
The way that email was set up for me is that the emails download onto my PC after auto ogging onto the Virgin.net server. That means, as far as I can tell,that I have all emails to/from my virgin.net account on my PC up to the point a few days ago when I got the 'authentication denied' message. Curiously, my wife aso as a virgin.net account and that still works fine on the same PC.
In view of the advice on this forum, we're both taking steps to use alternative email accounts etc. as it looks like only a matter of time before my wife's account is affected too.
I suppose that leaves one question - is it possible to close a virgin.net account to prevent unauthorised access or spam use by hacker etc and if so, how is that done given that neither Virgin Media nor Talk-Talk want anything to do with virgin.net accounts?
Wish I'd found this forum earlier, what a helpful and friendly resource it is!
Thanks