Thanks for a very comprehensive and useful reply. Yes I'm in the UK.

I suspect that my GP - who very much sticks to the book - would look at CRP and ESR first and probably refuse to do anything else if they came back within range. My CRP was within normal range on previous bloodwork. Do you know how difficult is it generally to get a GP to give you a plain film SI X-ray?

Not diagnosed. Lower back pain 30+ years.