Depending on your individual circumstances, you may need to declare your earnings to HRMC if they exceed your annual Personal Savings Allowance (PSA). Introduced in 2016/17, the PSA gives all tax-payers a tax-free savings allowance. If you're a basic rate tax-payer, you can earn up to £1,000 per year tax-free. Higher earners can earn up to £500 per year.  

You may need to consider paying tax on earnings for the tax year during which the interest was applied to your account.