Thanks for the reply.
I haven't tried uninstalling or reinstalling yet as reloading the aircraft seems to fix the issue, so I believe everything has installed correctly to the correct locations. I did run the installer as Admin originally.
I installed outside of my P3D folder in my Addons Folder though and notice that some of the fallback lines in the texture.cfg file point to locations within the P3D folder structure BUT only IF the install was made to the P3D directories. The lines I refer to are:
fallback.2=..\..\..\..\Scenery\Global\texture
fallback.3=..\..\..\..\..\..\Scenery\Global\texture
When installed to the addons folder, these two lines do not point to any existing folder. I'm not sure if the textures used from these folders are related to button textures as individual files aren't specified in the fallback lines. However, with some experimenting I have discovered that reloading the aircraft via the Vehicle > Select menu seems to fix the issue, so don't think this is affecting the issue.
Reloading the aircraft is a little frustrating, but a plausible work around.
Cheers
Adam