We usually obtain the retirement visa extension using the proof of income with a letter from the British Consulate costing 2000 Baht

This year if I renew the extension six weeks early with just over 800,000 in the bank I can avoid paying the 2000 Consulate fee, leaving the renewal any later might stretch the finances,will they allow this and will Immigration accept that the 800,000 is divided in two Thai banks providing that I get letters from both banks.

Thanks

Two banks is not a problem but six weeks early without travel/medical reasons may be. But nothing lost in trying and if it fails you have plenty of time to obtain letter.

I believe that a retirement one year extension can be applied for up to one month before it is due. I think you would be asked to show a good reason for doing it earlier than one month.

Most immigration offices will process extension of stay renewal within last 30 days of permission to stay. Chonburi is an exception and will process within last 60 days. But, as said, can often be renewed outside timescales with good reason/proof.