Under Self Assessment most people are due to pay their final tax bill by 31 January after the end of the tax year. Your tax calculation shows the amount of your final tax bill. Some people need to make payments on account in advance of their final tax bill. The payments on account are based on their tax return for the previous tax year and are due on 31 January during the tax year and 31 July after its end. Your tax calculation shows the payments on account due, if this applies to you.
Interest is charged from the date the payment was due.
There are exceptions to this surcharges are not imposed on unpaid payments on account, although interest will be charged. But payments on account which have not been paid by the time the final tax bill is due, become part of that final tax bill and are then liable to surcharge in rare circumstances we may not impose a surcharge or may reduce it, for example where we have agreed in advance to accept payment over a period of time. If a surcharge is imposed we will send you a formal notice telling you so. The surcharge will automatically be added to any other payments due from you. We charge interest on the surcharge if it is not paid by 30 days after the date of the notice.