Child Benefit 20132-13
I received this letter, too.
The letter has no contact details for HMRC, telephone number, or e-mail address.
The only option was to spend time re-opening the return and posting a comment in the "Other Information" box explaining why this charge did not apply. A complete waste of time, and some 3 weeks later still none the wiser if my note has been read, let alone accepted. Still awaiting a refund.