Tax code change for underpaid tax which is £0??

So I got a letter from HMRC saying my tax code was changing but not why. I noticed that my most recent payslip had changed the tax code from 1257L to 1117Lw1m1 - which I believe is a 'sort-of' emergency tax code of sorts. So it seems HRMC perhaps need to take tax that has either been underpaid, or some change in circumstances to tax for. Which would be the case except for anything outside of my PAYE from my employment, I do self assessment and paid in full with nothing owing for 2022-2023 tax year. To add more information on my situation and provide more clarity - specifically to my 2023-2024 tax year - as it involves both PAYE and Self assessment I have a full time job that pays ~£54250 a year and put 10% into a pension before tax. 23-24 was first tax year I had private medical through that employment - so got the P11D and worked out the tax which is £480 and set up to pay that in SA. I did some side income that was worth £1550, claimed trading allowance and have to ~£220 tax on £550 of taxable earnings - also put this in the SA. I earnt about £650 interest while holding a lot of money as primary executor for my parent's estate, so also put this in and owe about £60 in tax - again: added to SA. My wage also came with a £6600 sale bonus last year which - along with the side income, interest and medical insurance took my total earnings in the region of about £58000 or there abouts. My wife claims Child benefit so I am liable for about £1700 HICBC which also needs to be paid - again: added to my SA for 2023-2024. So all in all I ower about £2550 through Self assessment for the 2023-2024 tax year. But before I pay this - I would like to know why my tax code changed? It feels like HMRC are trying to claim tax owed on either the side income, or the medical insurance, or possibly something else. I did initially think I must not have paid enough tax in the 2022-2023 tax year and so HMRC were balancing the books. And I got a notification of such from HMRC while doing my self assessment - which makes sense..................until I say how much: I owe £0.00. If I owe £0 in underpaid tax, and have paid my self assessment in full for 2022-2023, and have done my SA for 2023-2024 (which I will pay when I clear this up)..........I am at a complete loss as to what the tax code amend is for. I suspect I am potentially being double taxed on one of the elements I mention above - but without any information I don't know :D Could it be underpaid money from my perm job for 2022-2023? I did ask my employer about this - but they said their books were correct and they just change tax codes when HMRC tell them to so...........................can anyone here help me? Anyone I can call? Anyone? Bueller?

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