Well after further googling, I discovered a downloadable tool from the NIST which evaluates STEP files. It is very awkward, you have to tell it the file in question, click a button that says "generate spreadsheet and viewer", and it will create and open 2 files: a spreadsheet showing the contents of the STEP file in only slightly more readable format than the text itself, and an HTML file openable in any modern browser that contains a rotatable, zoomable model of the contents of the STEP file. It mentions any errors it finds.
Thanks for this, CAD Assistant looks very useful! I have determined the file IS incomplete or corrupted using the NIST software but this is a lot more powerful, and runs locally too.
