@Richard *******
That was your problem if your marriage is not registered at your local Amphur. When you get back to the UK start this process :
1. IN THE UK: Marriage certificate legalised by the FCDO in Milton Keynes
2. IN THE UK: Marriage Certificate AND the Apostille from the FCDO Translated
3. IN THE UK: Marriage Certificate AND the Apostille from the FCDO AND their translations verified by the Thai Embassy in London
4. IN THAILAND: All the above (i.e. all 4 documents & their translations, making 8 in total) Legalised by MFA in Bangkok
then go to the Amphur