Thanks especially to heliograph I am now finally getting somewhere, by reformatting the pictures to an approximation of their final size on the paper before compiling the PDF. It's slow going, because I'm having to edit the file repeatedly for each picture, but it looks like the results will be worth it. I've shed about a fifth of the PDF file size by resampling the cover and illos on the first 13 pages, should be a lot more by the time I'm done.
The on-screen quality is pretty much as good as it was, print quality is a little poorer for pictures but still okay; jaggies if you look closely but not particularly bad, about what I'd expect for a native 150dpi picture - should have realised something was wrong when I saw how well the art was printing the first time around.
Anyway, many many thanks to everyone who has helped with this; hopefully I'll be making the big announcement some time in the next week or so - hopefully before Worldcon anyway.
Later Done! Final file size under 6 mb for 70 fairly graphics-intensive pages, which isn't too bad. Looking very nice, and I've submitted it for publication. If you're one of the people I asked to check it, you can download from the same address as before - same bat user name, same bat password. Depending on how soon I get any problems sorted, I may be able to announce it properly this week.
Later still Done! (again) and working properly this time - views correctly on macs and PCs, and a rather nasty little glitch that was stopping it from printing some pages is now fixed. File size is down to 4.54 MB, which is also good news, and if anything the print quality looks better.
I wish I could claim credit, but in fact someone else has fixed it for me...
If you looked at an earlier version the replacement is now on line, and I'd be grateful if you could find time to download it yet again, just to make sure that there isn't some other bloody problem.