Underpaid tax for earlier years included in 2022-23 tax code

hi This is the first time i have had to do a self assessment tax return. I am confused about the following questions: Underpaid tax included in PAYE coding (Page 1 of 2) Amount of underpaid tax for earlier years included in your tax code for 2022-23 (from your PAYE Notice of Coding). Please enter '0' if there is nothing shown I assume a "PAYE Notice of Coding" is an official document that i should be able to find under my HMRC log in. I cannot, nor can anyone at HMRC's call centres or webchat, clarify the existence or location of such a document. Under "HMRC services / Messages" there is a message dated 30th January 2023, stating that i had indeed paid too little tax on the year 2021/22 (by c.£1k) and that: "We are already collecting part of the tax in equal instalments by increasing the tax deducted from your current wages, salary or pension and we will continue until the balance you owe is repaid. Alternatively, you could pay the tax you owe now." Is this message a "PAYE Notice of Coding"? it is not clear that it is. It is also not clear how much of this underpaid tax i paid out of the year 2022/23 and how much was paid after April 2023 and therefore in the 2023/24 tax year. If i enter the full amount into my self assessment against the question above then, then it immediately adds on the full amount to the tax that must be paid by 31st January. However, I suspect some of it, possibly all of it has been or will be paid from my PAYE in 2023/24? How can i be sure of this?

Hi James Harwood,
We would need to access your record to check your tax code to see what the entry should be.
You can contact HMRC at Self Assessment: general enquiries
Thank you.