Your advice worked perfectly for me ! I created a Custom field under Action as you described and then used this for the ordering in my Export script. I have now managed to export my playlist into Excel and the tracks retain the same ordering as in the playlist which is exactly what I wanted.
I would NEVER have been able to figure this out alone though - so thank you for your help ! it might be an idea to include some information about this in the HELP manual. The software seems very powerful and flexible but I suspect you have to be a professional IT programmer (which I am) to be able to use its features.
Also I wonder whether making a change to the actual files is the best approach for this problem - I would have thought that when you do an export from the Playlist screen it should automatically retain the same "view" from this screen and allow 3rd party tools (like Excel) to do the customising. Alternatively you could include the Export logic but have an override so that if no Export script is specified the default is simply to export exactly as shown in the Playlist.
Below is my Export script,
build on %_date% with %_app% - the universal Tag editor - http://www.mp3tag.de/en/