Self-Assessment:- Failed to include an expense for my 2023/2024 year.

March 2024 I took out some business insurance on my self-employment, but I failed to add it to my expenses for the 2023/2024 year. This was a stupid mistake. I assume that the only way that I can include this in my expenses is to write a letter to HMRC explaining my mistake? I cant simply add this expense to my self-employment on my self-assessment for 2024/2025? Most of the insurance will apply to the year 2024/2025, but I took it out in March 2024. Taking everything into account, I think that it is best I simply accept the mistake of not including this expense, and so pay the extra tax. It will amount to just over a £120.0. The hassle would probably not be worth it trying to get it back. I will just learn from my mistake. Any comments? Thank You.

Hi,
Please find guidance here:
Self Assessment tax returns - If you need to change your return.
Thank you.