Thank you for your help. that solved my problem.
I have also now changed my URI
_api/Web/GetFolderByServerRelativeUrl('Asset Projects/Test_Project')?$expand=Files,Folders/Files,Folders/Folders/Files,Folders/Folders/Folders/Files,Folders/Folders/Folders/Folders/Files,Folders/Folders/Folders/Folders/Folders/Files,Folders/Folders/Folders/Folders/Folders/Folders/Files
This returns the contents of the folders down to repeated nested subfolders.
How do i get it to return the sharepoint library ID (not the unique ID) ? I need this so that I can add an action to update file properties for each