Hi,
You have two years from the due date of a tax return in which to amend the tax return, for example, the return 2022 to 2023 is due by 31 January 2024 and you have until 31 January 20026 to amend the return.
Where you have passed the two year deadline, you will need to write to, H.M. Revenue and Customs, Self Assessment, BX9 1AS and advise of the amount that has exceeded the tax free threshold. Assessments will be raised for each tax year and added to your Self Assessment statement. These amounts of unpaid tax will attract late payment penalties and interest, starting from 31 January through to the date the tax is paid. You can calculate interest and penalties up to 5 April 2022 here:
Calculate interest and penalties for tax years ended 5 April 2003 to 5 April 2022
Thank you.