A fairly unscientific test suggests that Proxy 62.255.83.11 is having problems, while most of the others in the Poplar pool seem to be acceptable - bad luck for sites that get cache-6 - notably some (maybe all) of the
www.wikia.com wiki sites.
If NTL could diagnose and substitute or rebalance automatically, then maybe their procies wouldn't have such a lousy reputation - the proxy issue is often quoted as a reason to find another ISP that either doesn't force proxy, or at lest manages without always having a proxy problem somewhere - if a route goes down, it reroutes as best it can, but if a proxy is bust, you're stuffed til they fix it.