Since January 2013 if your net adjusted income exceeds £50,000, you may need to declare a charge to claw back any child benefit that has been received by you or the person with whom you were living at the time the child benefit was paid.
This will involve also checking whether your partner has net adjusted income of a higher amount, in which case the partner will be liable for the charge.
HMRC are in the process of writing to taxpayers where they believe earnings are in excess of £50,000 and where child benefit had not been opted-out and the tax charge has not been paid.