Field Report: Chiang Mai Immigration.
Went this morning on the last day of my extended visa exemption.
Did NOT have my letter from the British Embassy.
Was issued 20 days "under consideration".
British folks who did have an embassy letter were given the same 20 days.
I would recommend having the letter as they did ask why i didn't have one. Nevertheless, the same result.
TLDR : Answer Summary
A report from a visitor at Chiang Mai Immigration indicates that he received a 20-day extension on his visa exemption, despite not having a letter from the British Embassy. Others with the letter received the same extension. It is advisable to have the embassy letter, as questions were raised about its absence.