Baffled that no-one else has raised or perhaps it's my research that's at fault. Either way.
I'm running 2.4.5 and whilst I can drag and drop in Files & Images, the actual sort order isn't saved. So the next time I visit the folder the contents has fallen back to it's previous order, presumably determined by the last modified date. I notice that the directory does pay attention to SortOrder though. I ran a bit of sql on the table which worked well and allowed me so sort the contents alphabetically.
UPDATE `File` t, (SELECT @rownum:=@rownum+1 rownum, file2.* FROM `File` file2 WHERE file2.ClassName = 'Folder' AND file2.ParentID = [PARENTFOLDERID] ORDER BY `name` ASC) r
SET t.SortOrder = r.rownum
WHERE (t.ID = r.ID);
It's not ideal, is there a work around? Is there even anything wrong? Thanks everyone.