Be clear what a default is - it is a record that you were late with making a due payment. If you make payment against a default, it will not clear from your credit file but be marked as settled - and rightly so. The story this tells lenders is that you're not a reliable payer, can be late and only pay-up when it affects you.
If you don't believe the default is correct, or that the debt it relates to is invalid, you need to dispute it - not pay it. Collate your documentary evidence that the debt is invalid, and send that to creditfileamendments@virginmedia.co.uk - they should investigate on a 21day basis and remove any entry that they agree is invalid.