Hello, I'm Keith and happy to help you with your question.
Do you mean HMRC's statement rather than your tax return form? Do you have other income and that put under the wrong heading by your tax office. Alternatively do you have pensions in issue or income from other sources incorrectly classified?
As I am in my eighties, and unemployed (for at least twenty years) I am puzzled by a tax demand related to employment -I hope the tax will be deducted at (the imaginary) source.