Well, for anyone interested, the outcome, after many private messages back and forth with Virgin Media Support people, is that Virgin have completely erased the old number and, not automatically but only after request, "offered" to refund the money that was being held in the account. They say they sent messages about the removal of PAYG accounts, but nothing was actually sent to the associated email account, so I can only assume they just may have sent messages to a phone which was switched off.
All in all, dreadful service, Virgin.