Revised tax code demanding a huge amount more tax - despite being PAYE

This year (2022 - 23 tax year) I had to pay the 'Higher Income Child Benefit' charge. That meant I had to complete a self assessment tax return. The outcome online was that I owed just over £300 for the charge. Today I receive a revised tax code. This starts off with £12570 standard tax code, then takes off £3060 for 'Child Benefit Charge' then a further £4862 for an adjustment for the current tax year. Then at the bottom of the letter it says the £4862 reduction in the tax free amount it because I owe an estimated additional £714 in tax!. I it is not clear how these figures have been reached. If I hadn't hit the High Income Child Benefit threshold, I wouldn't have had to fill in a tax return and all would have been well. Instead, I have filled it in and now I am being charged £700+ for some reason. I think I understand the £3060 for the child benefit coding - there's only 4 pay periods until the end of the year, so a portion at 20% and a portion at 40% divided by 12(ie per month) * 4 would be approx £300 - ie a big tax code change because the £300 is recoverable over a third of the year. Would the 'estimated' £714 be HMRC trying to collect this years (23/24) HICB as well - again over a 4 month period? I'm really confused and there is no email address or anything to get hold of HMRC - just a phone number, which isn't great for talking through / highlighting figures.

Hi,

When you filed the 2022/23 tax return the system will have picked up the child benefit charge and update your tax code for 2023/24 to collect tax at source on the charge going forward so that when you file your 2023/24 tax return you will have paid the charge at source so should not have a large balance to pay.

The estimated underpayment would be due to the child benefit charge being coded part way through the tax year. If you would rather pay the child benefit charge through your Self Assessment at the tax year end you can contact HMRC to remove the charge. 

Self Assessment: general enquiries

Thank you.