I think I've just hit this with extras.ubuntu.com .. a bit of poking around and it seems the address apt-cacher-ng was trying to connect to as seen with netstat was not the same as returned by 'host' at the command line. It occurred to me that the IP address of the server may have changed, but still being cached somehow by the process. So I restarted the apt-cacher-ng service and that seems to have cleared the problem..
