Error on self assessment

I have filled in my self assessment and it says I owe thousands of pounds in capital gains. I have already paid this but it hasn’t been registered on my self assessment. With the helpline being closed how can I rectify this so hmrc know I have already paid

Hi,

If you submitted your tax return online, you can log in and amend the return to show the gain in the box named:
"Total gains or losses on UK residential property reported on Capital Gains Tax UK Property Disposal returns: (optional)" and the tax already paid in the box named:
"Tax on UK residential property gains reported on Capital Gains Tax UK Property Disposal returns already charged: (optional)".  
On SA108, you would complete boxes 9 and 10 on page 1.  
You can also contact HMRC by using webchat at Self Assessment: general enquiries.

Thank you