Talk to your bank officer about this. Each bank has different policies on writing this type of letter and what they will sign. The bank letter is required if your savings rather than income are being used to meet the minimum income for support. If savings is used rather than exceeding the minimum income requirement then savings must equal 5 times the minimum requirement. That said, I provided a letter from my back anyway along with 6 months statements and my income information just to show I had the ability to support her. It is always better to have more than not enough. I believe the more given initially, the easier it is at interview. Best of luck!