Submitting the Tax Return and Paying the Tax
The deadline for filing the tax return and payment of tax is 31st January

Regardless of whether HMRC issue the Notice to File or not, you need to submit the Tax Return to HMRC by 31 January after the end of the tax year to avoid late filing penalties.
Late filing penalties increase significantly over time and may be based on the tax due after 6 months.

The tax and national insurance is due for payment on the same date as the Tax Return is due for filing (31 January after the end of the tax year).
Interest is charged by HMRC on all late payments.
There are also surcharges for late payment, in addition to the interest and late Tax Return filing penalties, and they continue to grow the longer the payment is late. It can get very expensive, very quickly. The late payment surcharge is 5% of the tax outstanding at 30 days, 6 months and 12 months after the 31 January due date. A large CGT bill paid late could lead to a significant surcharge.
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Submitting the Tax Return
Payment of Tax and National Insurance