Thanks for the response everyone.
I had managed to stumble on editing in the 2014 project version for two days, so the option of going back to the cc project version was not an option (I did test the xml idea, and yes, only the basic edit re-links.)
The project has now finished finally, but during all the problems we tried many software/ hardware troubleshooting options - new user, more memory, opening project in another 2014 project, cleaning the cache, affecting the library, un-install-re install, and Adobe took control of the computer twice!
We can only assume the problem derived from up-converting the project to the new 2014 software as every other option was evaluated.