Thanks for the reply - glad to clarify.
The file deletion window pops up as though there are a lot of files to delete, but then it never progresses. So no error message, just the progress window that never moves.
They're stored in an archive folder at the root level of an internal backup drive (so not the same drive as programs, but not an external drive).
I downloaded unlocker and tried that (thanks for the suggestion) but when I try delete the files that way the program starts and lets me select 'delete', but when I do that the progress window for the program appears but then after a few seconds says 'Not responding'.