Hi BlueBerry14,
Your banks and building societies will contact us at the end of the tax year to inform us of any savings interest you received. If any tax is due on this figure of interest, we will contact you detailing options on how to pay this. This figure of interest is also used to predict the savings interest you will receive in the following tax year, which is then applied to your tax code.
You can read more here -
Tax on savings interest.
Thank you.