Thanks so much for your reply. I am getting jaw x rayed hopefully this coming week so we'll see what it shows up. It's really good to see that others have experienced this as the 2 vets I've had out to see my boy say they've never seen anything like it before, and no one on the yard has seen it...
Gosh, I'm so interested to read your post as my 9yo Idxtb is going through exactly the same. He suddenly started jerking his head quite violently to the left whenever he tried to eat hay. I've had the vet 4 times already and we are really no further forward. They've checked his teeth twice...