First advise, is don't go to the Thai Embassy London! Use the Hull Consulate. They will do all by post and will give friendly, helpful service (unlike the Thai Embassy in London). There website has a wealth of info on it: The Royal Thai Consulate of Hull
In particulare look at this doc: http://www.thaiconsul-uk.com/pdfs/Af...pplication.pdf

..and as Oddball said, ThaiVisa.com (but check what you are told as there are a lot of red herrings thrown around.

Good luck