The osteopath, Laura, has given me an early birthday present, as you may have guessed from the title of this post. It wasn't a trapped nerve, eureka moment, but I really didn't think it would be. She explained to me what she reckoned was wrong, and it all fell into place.
The torn muscle is not all of the problem, and in fact was probably caused by a previous problem. I've always had problems with my back and neck, leading to bad posture, favouring one side of my torso, being constantly out of whack. Laura thinks this is what is lying behind the other stuff, and it makes perfect sense.
I'll have to go further later, as I'm about to take the dog out for a walk. Because I feel like moving again.