In this case TalkTalk are right, they have never, ever had anything to do with the virgin.net email addresses - it’s a sort of urban myth which permeates VM’s call centre staff like a bit of ancient wisdom and despite many, many claims that ‘it will be fed back’ and ‘call flows(?) will be updated accordingly, nothing ever actually happens and the same old excuse gets trotted out again and again. Still i suppose it does get the annoying customer off the line and they can chalk up another ‘successfully closed call’!
Now your problem is that you should have lost the virgin.net email address years ago, users who had the old dial-up or later ADSL services were given a year to either take out a VM broadband contract or find another email provider, after the year it was supposed to have been deleted. Now in some cases this wasn’t done on time and these addresses just lingered on. But as you are not a current broadband customer, there is no account to associate the email address with - hence it is regarded as orphaned and liable to deletion at any time with no further notice.
Probably this is what has happened to yours, now depending on how far the process has gone, it might, and I do stress might, be possible for the forum team to arrange for some sort of temporary access as a one-time only offer. However, you need to be prepared for the very real possibility that the mailbox and all the mail within has been irretrievably lost.