Supposedly you could go to:
http://www.virginmedia.com/contact-us.html
and pick Something else, which gives you two options for complaints. At which point you could, theoretically, make a complaint by Live Chat, which in theory is great because not only is it instant, it gives you a record you can save/print.
But like all things with Virgin I have experienced so far, the practice is not the same as the theory/promise - I have been trying all day, but always getting the message: "Sorry, the team are currently unavailable, please try again during our opening times.".
It is rubbish.