Have you looked through the full "Unlimited DVD Rental T&Cs" which give extra ways of contacting them? Easy to overlook because the page is headed up with four different T&Cs. I am at a loss to understand how they have managed to charge you without sending you any DVDs - I'm a member of Lovefilm, the largest one, have followed the industry and read the WHICH? report but have never seen this identified as a problem. Could this, from the T&Cs, possibly have something to do with the situation?
"By you: You may tell us that you wish to cancel your Subscription Package at any time by accessing the My Account area of the Site, and by clicking on Cancel Your Subscription Account."
If the worst comes to the worst maybe if you went into or phoned a major store?
Hope this is of some help.