Could not calculate the upgrade error

[alanrlow]

I keep getting this error message when update manager tries to upgrade my system.

Not all updates can be installed
Run a partial update to install as many updates as possible.

Then I get:

Could not calculate the upgrade
An unresolvable problem occurred while calculating the upgrade:
E:Unable to correct problems, you have held broken packages.

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
This is most likely a transient problem, please try again later.

Trouble is I have been trying for a week now with no luck. Any ideas what has gone wrong?

Alan.