I changed the public setting for the parent category to download=denied.
But then for registered users it did the same and locked the option!
I'm afraid i'm going to ask for some more noddy advice!
On the upside, i could see the download as public, and it said i needed to be registered to download, but it didn't show the preview.
Edit: thinking about it is preview only for media files, my files are PDF's, and i wanted a publicly available preview of the files (i have created another PDF for the file which has a snippet from the full thing)