After discovering the body, Mr. Martin is shown waiting to speak with Morse sitting in the back of a police car with his window halfway down. When Morse finally comes out to meet him, Mr. Martin exits the vehicle, even using the half-open window to close the door. However, when he turns to speak to Morse, the window is now fully closed.
The letter from Al-Jamara Education Department is seen to be dated May 3rd, 1986, and Morse reads it out as "May the third, this year." He makes deductions based on a mistake that a date referenced, the 21st, is a Saturday instead of, as Morse says, a Friday. But in 1986, May 21st was a Wednesday. The next Friday the 21st wouldn't be until November. It's possible that the dates were simply copied directly from the original novel, which would have been about a different year, probably 1976.