DB Update failed, database locked

Every day at around 0630 GMT I get output from the daily cron job:

```
Cron <root@racebest-live> test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
```

Reporting:

```
/etc/cron.daily/apt:
DB Update failed, database locked
```

I have unattended-upgrades installed and set up if that is of any relevance.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: apt 1.0.1ubuntu2.8
ProcVersionSignature: Ubuntu 3.13.0-57.95-generic 3.13.11-ckt21
Uname: Linux 3.13.0-57-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.11
Architecture: amd64
Date: Thu Nov 5 11:56:40 2015
InstallationDate: Installed on 2014-11-28 (341 days ago)
InstallationMedia: Ubuntu-Server 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.3)
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)