NOTE: If, after you have followed these steps, the port is still not accessible then you need to do a full reset of your Super-Hub and then try again. Very annoying but unfortunately this is what Virgin Media themselves recommend.
Go to the Virgin Media Super-Hubs administration website;
|Super-Hub: Main Administration Portal Logon Screen|
|Super-Hub: Standard Configuration Page|
|Super-Hub: Advanced Configuration (Main Page)|
|Super-Hub: Port Triggering Window|
Click "Apply" (making sure you've checked the "Enable" checkbox!) and then the port will be open.