An update. Virgin confirm the current situation is within their accepted parameters so they won't do anything. If and when overage reaches the required critical level, they would try to divert capacity from elsewhere in the network as a first resort. This would take at least a month after critical status is reached. Any permanent fix would probably not be effected until the end of this year.

My official letter of complaint has been lost and in any case will apparently not even be read, just attached to my account records.

They have offered me penalty-free cancellation. I will be taking it. Apart from the broadband issues, I find their TV service pretty lacklustre compared with Sky.