http://xmodulo.com/apt-caching-server-ubuntu-debian.html
After following this guide (which was easy) there look to be a few caveats
stick to the stable release as well as stable repos or you may start getting lots of 403 errors while trying to find local mirrors.
it took me a while to realise because i thought it was just a volatile repo but disabling the proxy on custom installs or weird builds fixed that problem.