@Martyn *********
You best get a list of requirements from "your" immigration before you apply. . . . . . . . . The “bank letter of guarantee” should be from the same day, for most immigrations. Just visit the bank in the early morning.
The "letter of guarantee" is called "rab roong thanakan" in Thai, it should prove 2 months seasoning (song duean) of a minimum of 800,000.- THB for your application to a 1-year extension of stay permit based on retirement. Immigration in Thai is called “Tor Mor”
Myself I let the bank prove THREE months of seasoning, just to feel safe in case a new immigration officer might not be aware of the two-months rule
When leaving the bank, draw a small amount from their ATM and keep the receipt.
NOTE A: sign all black and white copies in BLUE ink!
NOTE B: Immigration might check that you are TM30 registered at your place of accommodation
Since you are using the deposit method for the financial proof, you need to remind yourself:
After being issued the 1-year Extension of Stay Permit, a minimum of 800,000.- THB must remain on the account for three months. After the three months, the amount shall never go under 400,000.- THB.
For the renewal of the yearly Extension of Stay, the money must again be over a minimum of 800,000.- THB two months before the next application