You don't need bleed on the inside for saddle stitching. I always leave it at 0 on my own artwork, and prefer it like that on supplied artwork.
Are you saying part of the image is actally missing on the spine edge of the PDF? So if you have an image extending to the spine on a left hand page, there's an area of white inside the trim marks on the right edge?
thank you both. Indeed, when looking at the pdf there is an area of white and the printshop says it will print white.
Furthermore, if exported as a spread pdf, then the white spine issue disappears but then the cover and the back do not show as a spread and will be printed separately, according to the printshop and not match in the booklet and therefore I should bring the back and the cover into one spread. Is this possible? Will it not change the sequence of my pages? And how to do this? As ID gives this layout of first the cover and than the spread.
Presuming the print shop are taking care of imposition, I'd strongly advise against supplying your PDF in spreads... especially not reader's spreads.
Could you post screenshots of an effected page, in both InDesign and the resulting PDF?