I've had a pain in the left hand side of my chest for 6/7 weeks and today it was severe. Lying down on my front really hurts and I get pains in my back?

Chest wall pain? This is chest pain related to the muscles and bones of the chest wall. It can be caused by trauma (even lifting something heavy), but should be improving after 6-7 weeks. There is also a pain syndrome where the ribs become loosened from their fibrous tissue connection and "move" impinging on nerves. Or a stress fracture? But typically there would be pain with breathing. Surely time to see your Doc.
Chest wall vs other. The pain you describe sounds more like a chest wall pain in the rib joints. Did you get hit or cause pressure to the wall for a period of time. If it persists should see a provider to make sure nothing bad. If you get short of breath or nauseated with the pain then also should check with Dr. or be seen. Warm moist heat and Ibuprofen will sometimes help but check it out if not better. Hope helps.