I am looking for a specialist to consult about the possible origin/treatment of severe back pain which prevents sleep, and makes life utterly miserable.

After viewing x-Rays, Bumrungrad doctor advised it was not bone-related, and "didn't know what it could be"

He added that an MRI scan would involve 45 minutes in the narrow tube, and "probably show nothing"

Grateful for any recommendations - thanks.

Before you go looking for a pain specialist (of which there are not many, and they are going to send you to an otrtho anyhow, not treat you for pain of unknown origin) I suggest youy consult Dr. Wicharn at the BNH Hospital Spine center and see if he can get to the bottom of what is causing the pain.