Payment of Underpaid tax for 22-23

My Self Assessment (submitted on-line in November 2023) indicated that I have a small amount of underpaid tax for year 22-23 (circa £300). I did not check the box on the relevant question (Q2 of the Finishing your Tax Return section) and therefore, as in previous years, expected that this would be automatically deducted using my tax code. I am now receiving email and letters advising that this amount is due and must be paid by 31st January 2024 but no explanation as to why it's not being taken via my tax code as it has been in many previous years. Do I need to pay this or will it be taken through my tax code ? If I need to pay this by 31st January 2024 could somebody please explain why and why this year is different to previous ?

Hi JPR,
We would need to check to see why the balance has not been updated to collect in your tax code and update if can be done.
You can contact HMRC at:
Self Assessment: general enquiries
Thank you.