I have followed the instructions to set my Update Manager to check for release upgrades, i.e In Software Sources -> Updates -> Release upgrade it is set to "Normal Releases". I am happy to go from 10.04 to 11.x then to 12.04 as recommended, but it simply will not find any upgrades available.
I have also tried running
sudo do-release-upgrade -d
Checking for new ubuntu release No new release found