Hi David. For the group record, I accidentally replied only to David.
All the \consists in this project were already commented out. I can remove them completely. Being new to Lilypond, would that have any effect?
Here's how I've got things working temporarily. I removed a "\tag #'formidi" from the cello part. This seems to have done fixed it. I was using #'written and #'formidi so I could notate <a d\harmonic> artificial harmonics and have correct a'' playback. I'll have to spend a little more time with the #'formidi code to see if something I did broke it in a way it couldn't report a human readable error.
Sam