3 hours ago, Andycoops said:

obtain a mini statement covering the period between the 12 month statement and your application.

 

Which immigration office is it that requires this, please?

 

3 hours ago, Andycoops said:

When I do mine I order a 12 statement, 200 baht and takes a week to arrive at the branch.

 

On the day of extension obtain a letter from BBL confirming that it is your account, update bank book and obtain a mini statement covering the period between the 12 month statement and your application.

 

This is done while you wait.

I think the expectation by Immigration is that the balance on the letter, should match the balance on the statement/passbook.
Whilst waiting for the statement to arrive, ensure you have enough cash on hand and don't have to make a withdrawal, then a mini statement shouldn't be required to explain the difference in the balances.

They should then match.