This R85 tax form is used to claim to receive your interest without tax taken off. Generally if you are a non-taxpayer you should use this form. This saves you needing to make a repayment claim in later years.
The current HMRC location for this form is as follows:
Once you complete the form you should send it to your bank or building society. If you receive your interest Gross but later have a tax liability you need to pay the tax.
You need to complete a separate form for each bank or building society.