Gordon, I recommend any of the schools that teach the union system such as AAA in Chit Lom. Also have a look at
http://thaiarc.tu.ac.th/thai/thcourse.html
for some others.
Jika, go to my blog at
http://philipstokesel.blogspot.com
- click on the elephant picture for a link to my free 'book' on how to start learning thai.