Pay Self Assessment - a week late

Does the £1100 penalty fee for late payment only kick in after 1 month late? I have a very large SA tax bill that I won't be able to pay until 5th Feb. The 'Estimate your penalty for late Self Assessment tax returns and payments' calculator only goes up to 21/22 and it shows that only interest is payable if I pay a week late (which is a lot cheaper than me getting a loan to cover my shortfall). But I just wanted to check in case there is more of a penalty for 22/23? Do I still need to contact HMRC or can I just pay the week late?

Hi,

There should be no late payment penalties if payment is made within 30 days of the payment due date.

Thank you.

This is for HMRC as I cannot seem to find where to post a question. I have been charges interest 22/23 which is only due Jan 24 and which I paid. Why and can this be reversed. I have no joy trying to find a direct email or even get anyone to answer the phone to address my query

Hi,
If the interest charged is on your Self Assessment account then we will only consider your objection when the payment is paid in full.
You can object to the interest charges by phone or in writing to:
Pay As You Earn and Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom.
Thankyou.