Hi Joachim,
Thanks for responding.
I sent the printers the PDF. The client doesn't want me to upload the whole catalogue PDF, as it's their first catalogue and commercially sensitive.
However, I've uploaded the original PDF as sent to the printer for the page I showed above, as well as for a preceding page - you'll see why when you see the scans of the printed versions. There are artefacts everywhere on the preceding page, both around transparent PNGs and basic graphic shapes.
(The scans aren't great again - I wonder whether it's down to the paper used by the printer, as usually our scanner is fine.)
I've also uploaded the Publisher file for the relevant two pages.
Thanks for your help,
Patrick
PS
I'm aware that there's a difference in opacity between the two drop shadows on the second page; I was checking the difference.