Bike Transfer

Bought a bike a couple of years ago,the guy a fellow Englishman,gave me the green book (his ownership was not a problem) with a written confirmation confiming the sale. Although I realised shortly later that was not how to do a transfer I had no problem re taxing it,just kept it in his name.He has since moved ,but still in the Pattaya area

Now I want to leave Thailand and sell ,it will become an issue when selling on,and I cannot trace him. Immigration will more than likely have an address but may not give me the information,any suggestions?