Having had a friend buy one of these secondhand via ebay, and ask my help in getting it sorted, I thought I'd share this nugget of information. VIRGIN MEDIA will NOT activate second user boxes. They are very nice about it, but very firm. They argue that the box may have been "interfered with".
You connect it, log into the router's home page, make your settings and log out. Next thing is your browser takes you to an activation page and asks you to dial 150. You navigate the menu and then you are politely but firmly told "NO".
So there it is, for my mate at least, it was a complete waste of time and money.
However, I know that there are far more experienced people than me out there; so, if anyone finds a solution, do please let me know.