Hi Paulie,
Banks and building societies send details of untaxed interest to HMRC after the tax year end.
This figure would be used as an estimate going forward in your tax code.
As you are not due to pay tax the interest is shown as a deduction in your tax code.
If your circumstances change and you were due to pay tax the personal savings allowance would then be included.
If you think your tax code is incorrect you can contact HMRC to review https://www.gov.uk/government/organisations/hm-revenue-customs/contact/income-tax-enquiries-for-individuals-pensioners-and-employees
Income Tax: general enquiries.
Thank you.