Your currently on a multi entry too ? . Shouldn't be a problem , you have an address in Thailand . And for your extension as it's your second one your ( 800,000 ) needs to be in your thai account x 3 months .
I have my extension on October 18th by coincidence
You won't need 800k in a thai bank yet . If your getting it in London , take bank statements showing your balance etc ... infact if you time it right you can get nearly 2 years without the 800k
You are correct .. actually to prove that I went for my 90 days a week ago , and the guy next to me who had been away like you was told , you never needed to come in because your 90 day starts again from your arrival , point proved 😉