new uk passport, will be exiting at land border (mukdahan, spelling!) and then getting new visa at consular there. ie exit on old passport enter on new. will I need this letter? by the way it took 9 working days to process then get new passport back to bkk. great service.

You use both passport on departure from the country. They will transfer your entry stamp in the old to your new one and stamp out of the country in the new one. You will not need the letter at Mukdahan.

thankyou.