Thanks for your reply.
I'm using r157, latest stable, with desktop version 'June 13th 2019: updated to Chrome 75 (NW.js 0.39.0)'.
I'm going to try and export from the browser version of C3 now.
I'm hesitant to post on github as I can't publicly post a .c3p (I'm under an NDA).
Thanks for your guide, I'll go through it if it doesn't work from the browser version.