Error 0x80070005, Access Denied
Quick search on Google threw up a few tech forum posts noting that various anti-virus software can cause this error. The backup has run successfully many times previously with Kaspersky active but I tried turning off Lavasoft's Ad-Aware and ran the Vista Backup agian. It completed successfully this time. Problem solved.
Useful link for similar problems is Common causes and solutions to Backup, System Restore, and Complete PC Backup problems on the Microsoft Storage Team Blog.
UPDATE: Got the same error again, first thought was that it was being caused by the folder on the NAS box it was backing up to having been set to . Changing that didn't help, turns out it wanted Kaspersky Anti Virus turned off.
UPDATE: Got the same error again, first thought was that it was being caused by the folder on the NAS box it was backing up to having been set to . Changing that didn't help, turns out it wanted Kaspersky Anti Virus turned off.