Update!

After a couple more weeks trying the pain did not resolve. I referred her on for an opinion from a consultant anaesthetist wit a speciality in pain management, who preformed an Wikipedia reference-linkMRI. Turns out she has a small lesion on her C3 and C4 nerve roots, and that this was being referred to the anterior chest by branches of the supra-clavicular nerve.

Seems my original hunch was right in terms of neurology, and in a letter back from the consultant he referred to the difficulty of diagnosing this injury, particularly considering the pain was the only symptom with no peripheral signs. She is being treated with local analgaesic injuection for the superficial cervical plexus

Many thanks for the help!!