Sweary McSwear Swear.
So it turns out that when you have multiple files in one file area the path is very slightly different, so the 3rd pdf in my list has the path:
and the 4th has the path
So the commenter above was exactly right - I can't simply tell it to look for the file name, I have to find out what number is given to each pdf.
Am going to try putting all the pdfs into a subfolder within the file resource, and see if that works without numbering each item. I'll keep you posted...