OK, all now sorted with the F-104, however I still get a CTD when trying to open the F-4 despite having gone through the same procedure.
You don't have to go through the same procedure. The F4 HAS an FSDT installer so, you don't need to update the Addon Manager after its installation
Not that makes any difference, because both the FSDT F-4 installare and the Addon Manager will download the same, correct, gauge, from the same location.
When trying to change to the F-4 I don't even get the rotating model in the window, it is just black. Selecting ok from there just causes a CTD.??
This shouldn't happen. It means you haven't installed the F-4 correctly.
Uninstall it, then download the FULL F-4 Installer from THIS site, and nothing else. It includes everything that is needed, no need to update anything.
The only time you need to update the Addon Manager, is when you run an installer for a Cloud9 product that doesn't have an FSDT updated installer. Not the case of the F-4, although it will not make any problems if you do it anyway.