Where to amend extra payment received after P45

Hi, I have filled my 2023-2024 tax-return online through self-assessment. This is the 2nd time I have filled a tax return in this country so I am still not feeling solid about some of the concepts and scared to death to make a mistake which would cost me later as an unexpected expense. After submitting my return the system calculated me some substantial amount of overpaid tax. This sounds reasonable because I have experienced a dismissal followed by a non-working period. Anyway, I applied for collecting the overpayment but it has not happened for a long while. I have waited for the time they stated to ask for an explanation and finally got an adviser on online chat. She said there looked some extra payment from the employment that ended with dismissal. Long story short, I was able to figure out what was wrong in there. Because of a dismissal I have received a P45 right after leaving them. After P45, when the quarter finished, I received some extra payment because of the quarterly bonus. This arrived with a payslip and is taxed. When filling my return I have put the amount from the P45 and never realised the later payment was not in it. The adviser said: - well if you amend your return to correct the figure and include the underpayment in the return where it asks for any underpayments included in 23/24 tax code them you should be able to try and claim again it takes 72 hours to update Me: is it termed as "any underpayment"s section ? - yes it will be near the end when it asks for bank details the underpayment is what you need and it And she hanged up. I need to amend my return but I cannot figure out where the "underpayments" near bank details are. It gives me options to amend an income and/or amend sections but I could not exactly say where I should add this extra payment. Thanks.

Hi. I'm just a member of the public, but I think this "adviser" has completely misunderstood and given you entirely incorrect information. I know the underpayments boxes that are being referred to here, and they are strictly for when tax due in respect of one tax year, ends up being collected via pay deductions in a different tax year. I am assuming from the way you wrote your post, that both the P45 and the later payment occurred between 6th April 2023 and 5th April 2024. In that case, it seems like a simple case of additional payment received after issue of a P45. The explanatory notes for form SA102 (the paper version of the employment part of the online tax return, which can be found at https://www.gov.uk/government/publications/self-assessment-employment-sa102) address this case: > If you received any payments after your employer gave you a P45 or P60, these should also be included in box 1. The "box 1" referred to is "Pay from this employment". Therefore, we have an official document saying that the correct way to fill in - or, in this case, amend - your tax return is to add up the total pay from your P45 and later payslip, and include the sum in the section of the return about this employment. Similarly, you would also add up the tax already deducted from your P45, and from the later payslip, for the next box. So, you need to amend the figures you have already entered in the employment section - not add anything in any new sections/boxes.