Immigration aren’t interested in what you used the money for just that it dropped below the minimum required 400k. In allowing that to happen you have failed to continue to meet the requirements for which the extension was granted. You’ll need to apply for a new visa and start over.
Happy Born Day Tod. Have a great day. Your willingness to share your extensive knowledge has helped countless people over many years. Keep up the great work👍
For what it's worth this is how it works for me from NZ (I assume it will be similar for UK). Get the marriage certificate authenticated in the UK. When you arrive in Thailand it needs to be certified, translated and legalized (in that order). For me the NZ embassy certify, you then take it to a translation service and then to Foreign Affairs for legalizing. You then take it to your local Amphur where you'll be living to get the kor ror 22. Contact your embassy in Bangkok, I'm sure they'll put you on the right track. Hope this helps.
Couple of alternatives to paying an agent to do the whole process (which isn’t very difficult). 1. Get an O visa before leaving NZ then when you get here you won’t need an agent to open an account for you. Transfer funds then go to your local immigration office, they will give you a list of required docs for applying for your extension of stay. Get them together and go apply. 2. Alternatively arrive visa exempt, use an agent to open bank account, then you can start the process of applying for the O visa yourself. Does involve a bit more running around but will save you on cost of visa in NZ (since it went from $120 to $800 last March!). You’re going to be paying considerably more to use an agent for the entire process but it’s your choice.