Correcting arrears Self Assessment Error

Had to do a self assessment for 2020/21 and because at 83year old I didn't understand the online procedure I made a mistake by missing off the tax I had already paid via PAYE on my pension income. Because this was all in arrears I am unable to correct and the demand is that I now pay this tax again with late payment penalties and interest being added. I have written asking for a recalculation be done but have had no response or acknowledgement. Should I pay the extra tax error now to avoid any further interest being added or will the interest be paused because of the dispute raised.

Hi,
HMRC allows 2 years from the due date of a tax return (31 January) for amendments to be made, either online or in writing.  
This means that the last day a 20/21 tax return could be amended was 31 Janauary 2023.  
An overpayment relief claim can be made for a further 2 tax years up to 5 April 4 years after the due date of the tax return.  
Overpayment relief claims must be in writing and:
-must clearly state that the person is making a claim for overpayment relief
-identify the tax year or accounting period for which the overpayment or excessive assessment has been made
-state the grounds on which the person considers that the overpayment or excessive assessment has occurred
-state whether the person has previously made an appeal in connection with the payment or the assessment
-if the claim is for repayment of tax, you must have documentary proof of the tax deducted or suffered in some other way as you may be required to provide -this at a later date - see SACM3015
-include a declaration signed by the claimant stating that the particulars given in the claim are correct and complete to the best of their knowledge and belief
-state the amount that the person believes they have overpaid.
Overpayment relief claims should be sent to
PAYE Self-Assessment
BX9 1AS
Guidance can be found at:
Self Assessment Claims Manual
Thank you.

I am requested to pay the tax immediately so although the amount requested is incorrect because I missed declaring £2021.80 already paid through PAYE. So the question is: Should I pay now to prevent further penalty and interest payments or can I safely wait till my claim for over charge has been reviewed.

Hi Brian Roberts,
If you pay the amount, this would stop any further letters and interest being calcuated.
When the position is updated we can refund the amount.
If you don't pay then letters would continue to be issued and interest would increase on the amount.
Thank you.