If you don't want to spend hours on the phone to tech support you could always try VM's newsgroup support. They are very helpful and normally you get a reply quickly.
For how to setup outlook express for newsgroups:
http://www.virginmedia.com/customers...setting-up.php
and the group you need to subscribe to is:
virginmedia.support.broadband.cable
Post on the above newsgroup with a description of your problems and see what they're got to say on the matter. If you do find out your UBR is oversubscribed. If available find out the VM reference #'s and any `subject to change' capacity upgrade or resegmentation dates. Also ask for a note to be put onto your account detailing the problem. Then get in touch with customer service and ask for a reduction in monthly subs to compensate for the rubbish service until the problems resolved. I suppose if you're really racked off with the service you could try and get out of your 12 month contract.