Further to comment #3 above it appears that the /usr/share/applications/defaults.list file is created from the /etc/gnome/defaults.list file at login. Thus any changes to the /usr/share/applications/defaults.list file only last for that login. To make the change permament the changes in note 3 above must be made to the /etc/gnome/defaults.list file and a logout and login performed.
Further to comment #3 above it appears that the /usr/share/ applications/ defaults. list file is created from the /etc/gnome/ defaults. list file at login. Thus any changes to the /usr/share/ applications/ defaults. list file only last for that login. To make the change permament the changes in note 3 above must be made to the /etc/gnome/ defaults. list file and a logout and login performed.