I take it your ex was driving your car under your insurance so cover was reduced to TPF&T while he drove. Did he have a car of his own. Did he have comp insurance in his own name, did this expend to driving other cars. If he had then why not try claim under his insurance.
You are now liable to pay both bills from the mechanic total 5,500. As you have received 1,800 your are out of pocket for 3,700. Unfortunately you will have to write this off to experience.