HI GUYS. Thanks for the suggestions.
OK, Firstly, it doesn't matter what artwork you place in the picture boxes with the drop shadows. If you just have the drop shadows on a picture box without any content, we still get CMY plates. However, to answer you question, we're placing Greyscale TIFFs.
Secondly, we've checked the CMY plates in Acrobat and there's nothing there. However, for some reason, the printer (who prints all of these books digitally) can't run these pages as black only and is rejecting them.
If I take these Greyscale pages and produce a PS file as a Greyscale composite, then distil that file, I get a perfect Greyscale output on one plate. Great you might think. Except that some of the pages in these books are colour and some Greyscale. So, I'm trying to reduce the steps involved here and ideally I want to run the whole job as one PDF (because we have about 400 of these books to do this year). It really does seem to be the drop shadows that cause the problem.
I read on another thread something about blend spaces and dropshadows causing a CMYK output but don't know if its relevant.
Any ideas?