Sophos Anti-Virus for Mac OS X: installation fails with error 'info.plist could not be found'

Issue
When you try to run the 'Sophos Anti-Virus.mpkg' installer file from the CID an error message is displayed:

Info.plist could not be found, installation failed

Also the Installer Window will display the title:

Welcome to the @% Installer

Sophos product and version
Sophos Anti-Virus for Mac OS X

Operating system
Mac OS X, v 10.2 and above 

What to do

Ensure that you are running EM Library version 1.1 or higher.

• If you are running an earlier version:
  1. Delete the Mac OS X CID
  2. Uninstall EM Library, then install the latest version. 
• If you are already running EM Library version 1.1 or higher:
  1. Delete the Mac OS X CID
  2. Uninstall  then re-install EM Library.