Cogent might if you lucky send a message to NTL that their customers are complaining and take the opportunity to sell them an upgrade to their peering
. Or they might confirm there is overuse which gives you evidence of it at least. I have never found an isp so hard to report this kind of problem to, even Bulldog who got treated harshly by ofcom and the press have a guy on forums @ adslguide dealing with speed issues, NTL are unscathed with much worse problems.