Warning shown when 'Automatically check for updates' is set to 'Weekly'

[Phill]

If the setting 'Automatically check for updates' in 'Software & Updates' is set to 'Weekly' then a warning regularly appears in the notification area:

"The update information is outdated. This may be caused by network problems or by a repository that is no longer available. Please update manually by selecting 'Show updates' from the indicator menu, and watching for any failing repositories."

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: update-notifier 3.168.2
ProcVersionSignature: Ubuntu 4.4.0-51.72-generic 4.4.30
Uname: Linux 4.4.0-51-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Dec 8 10:55:15 2016
InstallationDate: Installed on 2016-06-03 (188 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: update-notifier
UpgradeStatus: No upgrade log present (probably fresh install)