Upon trying to reinstall Zurich9 I get a message saying - This Product requires the FS9.1 update to be installed. It definately is installed.
If the installer says you don't have FS9.1, then you don't have it.
The issue is, you probably *think* to have FS9.1 installed, but in fact, you have a mix-match of some files (DLL, probably) of 9.1, with the main FS9.EXE still on 9.0
This will happen if you applied the 9.1 patch over a modified FS9.EXE so, the file will not be patched.
The "About" for FS9, also, will tell you have FS9.1, but that dialog is coming from a DLL so, you can still have FS9.0 EXE, and be told by the About that you have FS9.1
So, in order to really install FS9.1, you need to first restore your original unmodified FS9.EXE from the CD, and then re-apply the update. In certain cases, if you have a BACKUP folder in your FS9 folder, you need to delete it as well before running the update.