I FIXED IT...
Observing that the FSUIPC installer identified a Microsoft Flight SImulator 10 installation in the windows registry (despite never having ever installed the software), I opened up regedit, navigated to the entry and deleted it.
Subsequently, the installation ran perfectly and the FSUIPC4.key was created in my Prepar3d modules folder as expected.
I did previously install a FSX payware flight model that was used only for Prepar3d and it obviously created the registry fsx entry that brought everything undone... FSX has never been installed on the target machine - only Prepar3d. (BTW FSUIPC was looking for FSX in a folder called "my programs" created by the payware installation.
Perhaps a warning to anyone installing FSUIPC4 to Prepar3d after having installed / downloaded any payware flight models.
:)