I assume that there is no problem renewing my wife's Thai passport then travelling back to the UK using the visa in the old passport in her maiden name. Her passport expiry is very close to the visa expiry so we want to get the FLR application in on her new passport. She intends to travel back now as once the application is in she wont know how long it will be before she gets her passport back so she will renew her passport whilst in Thailand and renew it to her married name.

The airline are ok with this although the UK travel agent through a major fit saying she would have to renew the passport with her maiden name. I put them straight on that but I assume the visa is ok.

Yes, the visa is still valid even though the passport it is in has expired.

As her new passport will be in her married name but her visa and the old passport it is in are in her maiden name she should also carry her original marriage certificate and a certified English translation so she can show the airline check in and UK immigration that she is the same person.

The translation does not need to be certified by the Thai MFA; for UK purposes a certificate as follows by the translator is sufficient;

 confirmation from the translator that it is an accurate translation of the original document
 the date of the translation
 the translator's full name and signature
 the translator's contact details