It turns out that the reason why I was having problems was that I'd installed Ghostscript while trying to get one of the other PDF programs to work - PDF creator includes its own internal version and falls over if Ghostscript is installed elsewhere. Once I'd realised that, uninstalled everything, and installed PDF creator again it worked fine.
Yesterday's attempt on the iBook produced a 56mb file. That isn't too surprising since OSX optimises for print quality, not file size.
Today's attempt with PDF Creator produced a 17mb file - which may not sound wonderful, but I've added a colour cover which on its own is a 10mb file! There are a lot of internal illustrations, so even at 150 DPI I don't think it's too unreasonable. Still have to add a back cover, and some stuff I'm expecting to get from the other contributors to the project, and I want to try to do something to pretty up the headers and footers a little, but essentially the first draft is done.
Don't know how the file size compares with Distiller - anyone seen any comparisons?