Upgrade failing, unable to determine reason

[JamesH]

Despite Googling and trying everything I could find, persistently fails to 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.
"

Only Error line in log, the rest are debug messages.

"2018-04-10 21:28:55,829 ERROR Dist-upgrade failed: 'E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.'"

dpkg --get-selections | grep hold

Shows no held packages.
Have done :

sudo apt dist-upgrade
sudo apt upgrade
sudo apt autoremove
sudo apt clean

Still nothing.

So at a loss, hence reporting.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: ubuntu-release-upgrader-core 1:17.04.10
ProcVersionSignature: Ubuntu 4.10.0-42.46-generic 4.10.17
Uname: Linux 4.10.0-42-generic x86_64
ApportVersion: 2.20.4-0ubuntu4.10
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity:Unity7
Date: Tue Apr 10 18:45:32 2018
InstallationDate: Installed on 2014-05-14 (1426 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to zesty on 2018-04-10 (0 days ago)
VarLogDistupgradeTermlog: