I got backache last jan13, no pain going in leg, at the moment have stiff lumbar & pain in right buttock if sit more than 15min. i've mri.

Sounds Muscular: Have you tried local treatments like ice packs or had any physical therapy modalities like electric muscle stimulation? Mri will not always be positive soon after an injury, most back injuries resolve themselves in a matter of weeks with conservative treatment. Trying different meds including muscle relaxants as well as nsaid's such as Ibuprofen or aleve (naproxen) may give better results. See your doc.