Managed to get everything to compile to the phone again. Also, I can move about the place now 🙂
The big issue was around the legacy Google Cardboard SDK junking up the project. Completely deleting all SDK related files, old and new, then reinstalling the new was the quickest solution.
Once that was done, recreating key items like the events system and camera using the supplied GVR SDK components made life relatively straightforward.
Big thanks to synthercat for his wise guidance.
Check out the video below.