@Richard **********
if you entered Thailand on a 90-days Non-Imm-O Retirement Visa, you don't have to prove that the 800,000 THB deposit came from abroad, for the application to the 12-months Extension of the Stay Permit. This proof is only needed when you apply for the 90-days Non-Imm-O Retirement Visa on Immigration inside Thailand, after you entered visa-exempt or on a tourist visa. This process would be called "change of visa type". You probably mixed up these two different approaches to the Extension