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
Self Assessment: Employment (SA102) 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.
