I originally arrived in Thailand three years ago on a one year O visa, issued in Hull, to 'visit' my Thai wife. I then obtained a teaching job, but unfortunately my initial employer did not realize you could obtain a work permit with this type of visa, and therefore had it changed to type B. This has been renewed twice, but I have now decided to revert to type O for logistical reasons. Last month I obtained a 90 day single entry O visa in Savannakhet, but am unclear of the requirements to extend this for a further year. My teaching income (Government school) is slightly below the ฿40,000 per month requirement. Do I still have to show ฿400,000 in the bank for 2 months, or just the balance to top up my income to the required level? is anyone out there in a similar situation, or able to offer any advice?

You didn't need to get a new visa, but could have asked for an extension of stay based on marriage instead of based on employment.

For an extension based on your marriage you either need a monthly income of 40,000 a month OR 400,000 in a bank account in Thailand in your name only for 2 months.

If you can't meet that, you can apply for an extension of stay based on employment.

Thanks for the prompt reply, Mario. The reason I needed a new visa was that my work contract was for 11 months only, so the B visa expired on 30/9 - this is one reason I wanted to change back to O. Wouldn't an extension based on employment mean reverting to a B visa? Also I'm sure I read somewhere that a combination of income and savings could be used as proof of financial credibility (e.g. ฿30,000 per month and ฿100,000 in the bank) - is this not the case?

No that is not the case - only retirement offers the combination to meet 800k per year requirement. But you should be able to extend or work on a non immigrant O visa issued for marriage (if issued for retirement normally a work permit would not be available). The extension only requires a non immigrant visa so either B or O would be accepted.

Is it possible to use a combination of Thai income together with a monthly transfer from the UK to top up the balance? If so, what evidence is required to confirm these transfers - would evidence of the money coming into my Thai bank account be sufficient?

You could possibly get an income letter from the UK embassy to prove your income. If you have enough savings in the UK to equal the 480K total annual income you would only need to show them your bank statements. You should contact the embassy for more info.

Immigration might accept an income letter from embassy and your tax payments for your income from working.

Is it possible to use a combination of Thai income together with a monthly transfer from the UK to top up the balance? If so, what evidence is required to confirm these transfers - would evidence of the money coming into my Thai bank account be sufficient?

No a combination is not allowed.