mount-spcontentdatabase Error Upgrade session failed. 1 errors and 0 warnings

We are in the process of moving away from SharePoint 2007 (sp2), our ultimate goal is to upgrade to SharePoint 2013 however I am having an issue getting past SharePoint 2010.

Using the database attach method, having backed up & restored the content database. Mounting the content database produces the following error log:

[powershell] [SPSiteWssSequence2] [INFO] [19/06/2013 09:38:41]: SPSite Url=http://sharepoint2010.domain.ac.uk

[powershell] [SPSiteWssSequence2] [ERROR] [19/06/2013 09:38:41]: Feature upgrade incomplete for Feature 'PremiumSite' (Id: '8581a8a7-cf16-4770-ac54-260265ddb0b2') in Site 'http://sharepoint2010.domain.ac.uk'. Exception: The user does not exist or is not unique.  (Inner Exception: The user does not exist or is not unique.<nativehr>0x81020054</nativehr><nativestack></nativestack>)

Similarly Upgrade-SPContentDatabase <Database Name> results in the same error reports

Central Administration manage Database Upgrade Status reports: Database is up to date, but some sites are not completely upgraded.