You are always welcome Ian. If this is still of no help then please send us the .AIP (project file) and a verbose log of the installation to support at advancedinstaller dot com so we can investigate them.
In my case it seems to be a problem with the permissions of the folder %programData%\Microsoft\RSA\MachineKeys folder
Because, as suggested by Dan, I've tried to create the AppPool manually and it raises a "keyset does not exist" error.