Correcting Self Assessment (>1 year old)

In preparing my self assessment for 23/24 I realise I misunderstood how to account for accumulating fund dividends and cash based funds and have underpaid. I have read the guidance and will contact HMRC in writing as it is from more than a year ago. Q - when I declare the imcome, I presume I also provide what type of income (dividend, etc) Do I use any numbering system (as per the original self assessment?) Q - In the original submission I depended upon HMRC to make the calculation. In making an estimate of the underpayment, is reasonable to simply use the highest tax band applied to income? I assume hmrc will recalculate accurately. Thanks.

Hi,
Yes when you submit the amendment you will need to supply the details of the section/box that needs to be amended and the new figures. Once updated an amended calculation will be issued to you, but in the meantime you should estimate any balance that would be due and make the payment to limit any interest/penalties that may be due. 
Thank you.