Can't get updates

On July 22nd, a popup told me that I had 160 packages that needed to be upgraded. I clicked Remind me later" and I have since heard nothing.

The daily run of /package-data-downloader does nothing. How can I force it to try again?

I have read somewhere that I should try:
   sudo apt-get update # Fetches the list of available updates
   sudo apt-get dist-upgrade # Installs updates

The first of these produces a mdest number of 'Hit@s amidst many more 'Ign's, but then I get lots of errors of the form
   Err http://gb.archive.ubuntu.com quantal/main Sources 404 Not Found [IP: 91.189.92.201 80]
   Err http://gb.archive.ubuntu.com quantal/restricted Sources 404 Not Found [IP: 91.189.92.201 80]

When I try the second, I get a long list of packages, followed by
   156 upgraded, 4 newly installed, 0 to remove and 0 not upgraded.
   Need to get 258 MB of archives.
   After this operation, 205 MB of additional disk space will be used.
   Do you want to continue [Y/n]?
   WARNING: The following packages cannot be authenticated!
followed by a vast list, and then
   Install these packages without verification [y/N]? n
   E: Some packages could not be authenticated
and then it ends. Should I have answered 'Y'?

AFAIK, I am using Ubuntu 12.10 (yes, I know there is a more recent version, but I would like to get this sorted before proceeding further.