What is the profile assigned to the document? The fast way to check is Edit > Convert to Profile... and then cancel.
And are we talking about objects created in ID or that were imported?
The profile assigned to the document is the same - coated fogra39 etc etc. Its objects created in indesign - the main problem is the text going from black to full colour
ID should most definitely not be converting 100%K in a 4-color mix unless there is a change in profile. Are you seeing the change in your PDF, or only in the output from the printer? If you see it in your PDF using Acrobat's output preview, the problem is someplace in what you are doing, if not the printer is adding a new conversion and you need to find out what that is.
If you see the conversion in the PDF you should go back and make sure that the color space is really correct in ID in the file (Edit > ASSIGN Profiles [you want to assign, not convert, or you'll make rich blacks in this operation]) or in the Export settings (Convert to profile (Preserve Numbers)). If the profile is correct in ID you should be able to output either with colors unchanged (if there is no RGB or other out of profile content) or either of the Convert to Profile options, perserving or not preserving numbers, and your native objects won't change.
yes the four colours are in the pdf - when i use the output preview in acrobat the 'black' areas are rich black rather than pure key - thats useful as its definitely something my end rather than with the printers. I set up the document in the fogra39 profile but when i was creating the pdf didn't use the convert to profile (preserve numbers) option, so i guess thats where the problem is coming in.
The only reason that your 100% K should convert during export though, even if you don't use the Preserve Numbers option, is if the document working profile and the output profile are not the same, so you need to double-check your settings.
The profile assigned to the document is the same - coated fogra39 etc etc.
...when i use the output preview in acrobat the 'black' areas are rich black rather than pure key.
If you assign a profile Acrobat's Output Preview shows you the values based on the destination or Simulation Profile. So here I exported to Fogra39 and included the profile, but the Simulation Profile is set to my Acrobat working CMYK profile US SWOP Coated, so the numbers show the conversion from Fogra39 to US SWOP:
If I set the Simulation Profile to Fogra39 there's no conflict and I get 100% Black:
If you export to PDF/X-1a or 4 the Simulation Profile will default to your doc's profile when you open the PDF.