Decree absolute original,passport.

You need a freedom to marry cert from British embassy in Bangkok.

Then you need to have it all translated into thia, this will be explained to you in embassy and they give you a piece of paper to show steps cost about 4000bht to get it translated and stamped .

Then you take all this to your local district office and they will do marriage cert.

Hope this helps as I did this last month in chaing mai .

The British embassy in Bangkok is great and very help full you book your appointment online very easy. Hope this helps