Virgin were under no obligation to mention the return of their equipment when you phoned to cancel your contract. When you signed up to the service, either online or on paper, you will have been presented with their terms and conditions. These state:
"If we or you end these agreements, or you decide to disconnect from some of our services, you must return our equipment to us (or let us collect it from you if we choose to do this). You must return equipment or make it available for collection in a reasonable condition, allowing for fair wear and tear. If you fail to return or make available the equipment for collection for any reason, we are entitled to charge you for the replacement cost of the equipment".
http://allyours.virgi...al/oncable/terms.html
Unless you can show that £55 is excessive as 'replacement cost' (which I doubt) you're legally obliged to pay up. You can either do so or wait to see if they'll bother to pursue (through a 'small claims' court action) to get their money. (Some companies always chase every penny that they're owed, while others simply give up. The only way to find out what Virgin's policy is will be to 'wait and see').
Chris