Self Assessment and Tax Code Changing

james231287

Hi all,
I wondered if someone could help me - as I cannot get through to HMRC. Many thanks in advance.
My situation is simple in that I only complete the self-assessment as I have to pay my Child Benefit payments back - that is no other complexities in my tax return. 
2 weeks ago I completed the Self Assessment on the HMRC website and it gave me the figure I had to pay back to them by 31/01/21 (£1079) - No problem.
But today I've received a letter from HMRC saying they are changing my tax code to K102X saying that I Owe £425 - even though I opted to pay my child benefit back as a single payment (£1079) , rather than having it collected through PAYE.
A few questions:
(i) Can anyone suggest why HMRC may have done this?
(ii) To correct my code back to 1250L - is it just a case of calling them and telling them to change it back?
Thank you.
James

[Deleted User]

As an educated guess I suspect that they are collecting the £1079 as underpaid and preventing it from happening this year. The £425 is more than likely the amount that would have been collected in April and May had you been on the new code from the start of the tax year. In my day it was specifically the case that an underpayment could not be collected in the year that it was assessed but I believe that this has recently changed. If you want to speak to the try at 8am sharp.

sheramber

Under 'finishing your return' there is a box which says 
If you owe tax on savings, casual earnings and/or the High Income Child Benefit Charge for the 2020–21 tax year, we’ll try to collect it through your wages or pension by adjusting your 2020–21 tax code. If you do not want us to do this, put ‘X’ in the box - read the notes
Did you  put an 'x' in the box?

james231287

I certainly did put an X in the box. But thank you - I'll keep trying HMRC.
Today I called and after inputting my details was told, 'we are  unable to help you, goodbye' by the automated system!
Back on at 8am for me,
Thanks both for taking the time.