It appears that the latest Android versions, prevent most apps from accessing the more public regions of the phone's storage areas.
After trying several apps, most of which have forced advertising or crippled options, I'm happy to say that Marktor stepped up and covered what I wanted it to do.
- Save and edit .md files without any faff.
- Store and read these files from a location visible to all other apps.
Here you go download from the Android store and give it a try for yourself. Thank you very much Gregor Santner, please avoid changing this app!