What documents are required to register a marriage in Thailand for a UK citizen married to a Thai national?

It depends upon who certified your marriage certificates.. both the English version and Thai translation must be certified.. because we married outside of Thailand our original marriage certificate had to be certified by the Embassy in Bangkok.. then it was submitted along with the Thai translation to the MoFA for certification.. and the document we received was a combination of the 2 certificates stitched together duly stamped by the MoFA.. which we needed to register our marriage at the local Amphur.. we used an agent to avoid the hassle and cost of travelling to Bangkok.. which was well worth the 5k baht we had to pay them!

At our Amphur, they required a Thai translation of the passport. For good measure, I obtained a certified copy of the passport from the embassy (UK in my case) and then had this certified copy translated.

Depends on if it cerified by MFA,an agent got mine done for me at Chaing Mai.Ems postal service,cost about 4000baht door to door

Certified by whom?

Normally you need to get the marriage certificate apostilled in the UK and approved by the Thai embassy in the UK, then in Thailand you get it translated and certified by the MFA before going to the amphur to get the thai certificate showing you are married to use when immigration