Thank you for the response and help.
After several failed attempts, I have now been able to change my password (including an upper case letter as required) and log into my @virgin.net email account.
I understand from other threads that email accounts not associated with a broadband account are being deleted. Although I do have a broadband account, it appears that I have two different account numbers (in two different area codes) and neither of them seem linked to my email account*, or those of my family. Can we get all this consolidated into one account somehow?
*(For example, when I've tried the forgotten email route and filled in my date of birth and account details from my bill, I get the message "We've found you! We've found a number of email addresses linked to this account." but the drop-down box is empty!)