Follow up:

I have since had a cervical spine MRI and chest x-ray.

The MRI was performed to look for signs of C8 radiculopathy. Results were normal, and showed no spinal stenosis or focal nerve root compression.

The x-ray was performed to make sure there were no tumours (e.g. pancoast tumours) causing nerve compression. Results were normal in terms of neurology, though thoracolumbar rotatory scoliosis was incompletely visualised.

It appears my GP doesn't know what to do from here, so I have a referral to see a neurologist.

What would the neurologist offer me that hasn't already been done? I seem to be at another dead end!