[ubuntu] Unable to upgrade from 11.10 to 12.04LTS

Hello,
I have been trying to upgrade from 11.10 to 12.04 but i keep getting these error messages

W:Failed to fetch http://gb.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found
, W:Failed to fetch http://gb.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found
, W:Failed to fetch http://gb.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found
, W:Failed to fetch http://gb.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.


My first Ubuntu installation was 9.10(Karmic Koala) and I have always done upgrades till my current version 11.10(Oneiric Ocelot)
I think there is no more support for some Karmic packages and this is affecting my upgrade.
Can anyone help ?