Thanks for the reply.
I have discovered that the issue is caused by our installer package setting the registry value 'HKCU\Software\Adobe\Adobe Acrobat\10.0\Originals\bBrowerIntegration'=0
The user interface to this setting is disabled. When I change this value to 1 via regedit then all works OK.
I will need to get the installer customisation file rebuilt to correct this properly.