When I used get_iplayer to download a programme from Radio3, it was stored as a file with the name ending "original.dash.m4a". This automatically converted itself to a plain m4a file, which could be moved to another folder, edited, played, etc. But I cannot delete the original file, either from Windows using administrator privileges or from the command prompt. Any ideas?!

This is a problem outside the scope of get_iplayer.

Under normal circumstances get_iplayer wouldn't leave behind a file that has been converted which means there's something unique to your system preventing that. Probably the same thing that's preventing you deleting the file manually.

Google will give you many ways to try deleting the file but it would be worth you exploring what caused it in the first place but we can't know what that would be.