Hi Dave,
You didn't mention your age at all, however this could be impingement as your GP mentioned or any of a range of conditions, but perhaps the one which springs to mind is that of Frozen Shoulder. You can view more about this condition on our page at http://www.sportsinjuryclinic.net/cybertherapist/front/shoulder/frozen_shoulder_adhesive_capsulitis.htm
Can you get in to see a physio or sports therapist? They might be able to help you out somewhat and at least give you some advice? I have a leaflet that I give to patients about frozen shoulder if it'd be of any use?
Ruth Cheesley BSc(Hons) MSc MCSP
Chartered Physiotherapist