As far as I know the 30 day money back guarantee is still in place:
http://allyours.virginmedia.com/mone...guarantee.html
Plus, I Can't see VM having a used router back so:
If you got the router free, then I Think you'll end up paying £ 40 for it if you leave early.
If you paid £40 in the first place, I don't think you'll pay any extra (ebay time)
http://allyours.virginmedia.com/html.../wireless.html
The quickest way is to ring customer services and ask.
Laters,