I have spent several hours trying to repair this part and am getting nowhere. It claims to be a joined srf but exposing naked edges doesn't reveal anything. Does anyone know what the problem might be or have any advice on how to fix it? My next step is going to be to redraw the part.
Any tips would be hugely appreciated.

Your file opens as a solid over here.....

That is even more puzzling, when I exported the part it was a joined srf with no obvious issues, but now when I open the exported file it too is a solid. I wonder why?
Thanks for taking a look.

Although I brought your file into another app and created an issue with this surface here:




Try replacing that surface and see if it helps you out...

I have, very rarely, managed to create objects which have some strange "leftovers,"
which when saved to 3dm and reloaded, are OK. (I know, it is a vague statement:-)
- Brian

Sometimes exporting as STEP and then reimporting fixes issues. Make sure you have some sort of scale set in your preferences before doing it.