They should have a record of who had what IP when, but the beauty of virgin media is that cloned modems are so rife that its remarkably hard to prove it really was you, even if it was. A claim of 'Its BS, my modem was cloned' will require them to prove it wasn't, as this is a common thing, and will require a lot more effort than most anti piracy outfits will give to a case. With ADSL they can prove a connection, tied to a phone line, tied to a physical address. No such thing on the virgin network, from what I can see your IP is merely a sub address of the router for your area.