Tax allowances

Hi, My aunt is receiving a state pension of about £75 per week. She has £25,000 to put into the bank to receive extra income. I have 2 questions:

1. What is her tax allowance - I understand that the first £4,000 or so of income per year is not taxable...what is the exact figure ?
2. Has her state pension been taxed before she receives it ?

I ask these questions because I need to know if we can put away the £25,000 in an account and fill in thr R85 form which means she will pay no tax on interest...

Thanks in advance.