Had a cough and back pain for 12 weeks had xrays and bloods. All fine. Respitory specialist not concerned if I had lung cancer. Would it show on xray?

See below. Chest x ray would pick up large cancers but may miss smaller lesions; if you are a heavy smoker and the cough persists you should consider ct; however it appears u are 35 years old; if that is true it is very unlikely that you have lung cancer.
Often not. A ct scan would be a better test for lung cancer. Your story of cough and back pain in a young man is not typical for lung cancer, though.