These files are only able to be opened by InDesign - of the same version or later, as the version they were created in. Maybe you could reinstall InDesign, open them and export them as PDFs.
I have the whole Adobe Suite - just didn't want to put InDesign back on, since I never use it any longer....sigh...might have to just to convert a few files, and then take it off again. Thanks for your response.
Oh dear, at least you can access your documents when you reinstall InDesign. As a matter of interest which version of InDesign and which OS?