I had the same response from my server host(s) they traced the faults to within NTL's network, I have been struggling with NTL to try and get an answer for weeks now, and recently my problems have been escalated to network management, but I seriously doubt it will fix anything.
I had 2 placating phone calls, and was told to 'give it a week' but nothing has been fixed, the evening is terrible, I can't really use my servers at all, and neither presumably can a lot of other NTL users.
It seems the NTL network is pretty rubbish when it comes to web browsing, the proxies break so many sites its not true, steampowered.com is currently broken on loads of their proxies (mine included) which is just amazingly bad, and I guess it will take months to fix as usual.
I really don't want the hassle of leaving them, but I'm pretty much at my wits end too, the level of incompetence is stunning, Cogent bandwidth past 6pm is totally useless and has been for weeks, I think the tech staff must be morons
or if they are not then they are not being allowed to fix things which doesn't make any sense either !
I've an idea - every time you come across a site that doesn't work properly with the proxies, raise a responstek form, one for each site - maybe if they get hundreds of complaints they will have to do something.
Other than that or leaving I'm fresh out of ideas, they suck, badly.