Today the thing's flared up cos I overdid the lifting during shopping.
But the hand therapist is now doing twice weekly massage and manipulation and that's freed it up a lot. I could no longer improve the rotation with exercises without pain.
Finally found out why she was so concerned about pain. Seems wrist and ankle fractures can cause a pain syndrome that leads to muscles, nerves and the local vascular system to go into overdrive. That locks up the joint and there's no certain remedy.