unable to upgrade

[rebroad]

I'm running Ubuntu 12.04.5 LTS and I'm wanting to upgrade to Ubuntu 14.04.3 LTS.

Running update_manager, it first complains that "no valid mirror" found. I choose "yes" to rewrite my sources.list. Then I get:-

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This can be caused by:
 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu
 * Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.19
ProcVersionSignature: Ubuntu 3.2.0-88.126-generic 3.2.69
Uname: Linux 3.2.0-88-generic i686
ApportVersion: 2.0.1-0ubuntu17.9
Architecture: i386
Date: Sun Sep 6 19:00:03 2015
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Beta i386 (20110903)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-09-06 (0 days ago)
VarLogDistupgradeTermlog: