How are you trying to log the complaint? Via 'My Virgin Media' or a different route?
Complaints on the forums are often about how pointless it is making a complaint to VM (seemingly it is often a largely automated process with contested claims of VM being 'unable to contact you' and closing the complaint).
You may find making a post on one of the technical forums ('Networking and WiFi' or 'Speed') about your broadband problems might get you a more useful outcome than a complaint route.
One of the VM forum team should respond here within a day or two.