Including P800 Underpayment Repayment in Self-Assessment: How to Reflect Correctly?

1. I've been asked to prepare a Self-Assessment tax return for a previous year. During that year, I received a P800 tax calculation showing that I had underpaid tax, which I subsequently repaid. How should I include the extra tax repaid in my Self-Assessment form? 2. If the repaid underpayment from the P800 does not automatically show up in the tax calculation within my Self-Assessment, what should I do? Is there a specific section in the Self-Assessment form where I need to include this repayment?

Hi,
If you received a P800 and paid the underpayment and now need to file a Self Assessment then you will need to contact HMRC to cancel the P800 and arrange for the
payment made to be transferred to your Self Assessment record.
You can contact HMRC at Self Assessment: general enquiries
Thank you.