I have an old Virgin/Telewest SB4100 modem and have now found out that i need a FREE modem upgrade to cater for my 20mb line. Nice of Virgin Media to let their XL Broadband customers know. :(
To check yours go here : http://allyours.virginmedia.com/html/existingcustomers/faster/current_service.html
If it says you need an upgrade just fill in the form the webpage takes you to. You'll need your account number !