Paul, did you just complete your 12 month visa ext for a spouse? (Marriage ext). If so did you obtain a bank balance certificate at this branch that confirms the amount of money you have in the bank? If so its a Bangkok bank policy to freeze your account for 4 months normally before issuing the certificate. Maybe the 6 months is because its a new bank account or miscommunication.
If you complied with all the visa requirements then you should be able to do it yourself, if you didn't comply your immigration office may not want to take your application.
What about the Ministry of Foreign Affairs (MFA) office in Chiang Mai. The MFA provides legalization services at the passport office located in Chiang Mai International Exhibition and Convention Centre, 1st Floor, Chiang Mai Province.