If your having trouble with your system updated cause for some reason I do on Dodo Australia with the sub-domain mirror http://au.xxxx.ubuntu.com you can set them to use the main source which always removes the error for system updates in most conditions.
First load your System Settings it looks like:-
Then load the one highlighted in the image above called: Software & Updates; it looks like this:-
Now change the Source Code [-] Download From to : Main Server
That is all you should have to do; do an update in the terminal and the next update should work fine; the commands for this is:-
$ sudo apt-get update