Hawkwind48

After a lot of testing, I finally have success.
Using DesktopServer to make a copy of the site, I re-saved the permalinks and removed all plugins that I knew I would not need. Updated DM and migrated my 30 downloads with the system now reporting that I had 59 published downloads. 30 had no icons or categories and showed links with 4 digit codes. The remaining 29 came complete with icons and categories using 3 digit codes. I updated the 30 missing categories then uploaded the recommended button plugin before testing.
There was no change ie I still could not add files for download, add an icon or change the download script. I then deactivated every plugin, activated only the DM and the button plugins, then tested. Everything worked as expected. Continuing to test, I activated each plugin in turn and checked immediately after that I could modify the downloads by changing icons or text. Worked as expected. Interestingly, it only worked after a gradual reactivation and not a block reactivation.
I do not claim to understand what has happened or why it is now working. It was the complete deactivation and gradual turn on that seemed to do the trick.