Simple assessment & higher rate child benefit charge

[Dp73]

A friend has received a simple assessment from HMRC for 17/18 & 18/19. Due to an error in her tax code (which she had not noticed, due to lack of knowledge), she has to now pay £4k of tax which had not been collected through PAYE. She accepts her mistake and has made arrangements to pay.

After getting me to take a look, I asked whether she was claiming child benefit (for her 2 children), and if so, whether she had repaid any of that- her taxable income is £55k. Yes she is claiming, but she wasn't aware of the need to repay. If she declared this now, and with already having to pay tax due from the simple assessment, she would be in serious financial difficulty.

So my question is this: if Hmrc issue a simple assessment, should they have assessed your requirement to repay child benefit for that year, and so have no basis for demanding its repayment later? In other words, are your tax affairs for that year now closed.
(On asking this, I do obviously realise that some of the child benefit received should have been repaid, as stated above).

Thank you in advance.

[jerome.lane]

HMRC could still seek to collect the child benefit as basic PAYE adjustments won't draw a line in the sand. In fact, I'm surprised they haven't already as it seems a fairly automated system. Depending on adjusted net income, the amount repayable will probably only be about £500 per child (off the top of my head). Maybe your friend is contributing to a pension or gift aiding a small fortune and doesn't actually owe anything?

[Dp73]

Thanks for the reply.

Yes, you are right regarding the amount, but the potential for it to be backdated over several years is the concern- how many years can Hmrc go back?

£55k is her 'child benefit income', so after pensions, etc have been deducted.