Hello Gavin,
You may not be able to import recent versions of PDF file due to the ZIP and JPEG Compression methods in QuarkXPress 6.5 (they can be imported in QuarkXPress 7.x and 8.x).
Please try these steps:
1. Save the PDF without any compression methods.
2. Save the PDF to a lower version like 1.3 or 1.4 and import again. (You may save the PDF to a lower version from Acrobat Professional).