Thanks everybody for your input. The link is working now: PBR_JSON_format. The cat at that point had decided that it was dinner time and wanted pre-dinner treats so she took over the editing and typing! I will have words with her.
My objective here was to have a simple easily edited file that can be readily parsed by both C++ and Python. It looks to be pretty close to glTF. If you look at the sample file at the bottom of the link you will see that is consists mainly of file paths. Everything is text with the exception of the boolean value for skybox. I kept colours as names so that a new user can easily select from pre-defined palettes. I have been restrictive in that only a small set of keys are accepted and file paths are strictly checked. This is to make it easy on the user.
As you can imagine, the old versions were quite cumbersome with a very long command line (generally more than 400 characters) specifying at least five files and possibly an object.
Here, the user can easily edit the JSON file changing colours, switching off the skybox, adding new files etc. For example it is now very simple to switch to the six cube map files instead of the equirectangular file (-c) on the command line.