Thank you
I recently sent documentation to the British Embassy, Bangkok, requesting the "proof of income" letter, to support the extention of stay for my existing visa.
The website said, with the correct paperwork, the letter would be issued within 10 working days. I received it within 6.
To Chiang Mai Buddy for scanning my documents, to the British Embassy for their excellent service, and to the people on this group who gave me really good information, thank you all so much.