I believe the hon. Member is referring to the Court of Appeal case originally bought by the Child Poverty Action Group about whether the Department for Work and Pensions has the power to recover an overpayment under common-law in addition to their powers under section 71 of the Social Security Administration Act 1992 (SSAA 1992).
HM Revenue and Customs (HMRC) does not enforce recovery of overpayments of child benefit, guardians allowance or the health in pregnancy grant in cases where section 71(1) of the SSAA 1992 is not applicable.
HMRC’s powers to recover taxes are statutory. For example, section 28 Tax Credits Act 2002 gives HMRC the power to ask for any overpayment to be repaid. HMRC’s policy on how it uses this power is set out in its code of practice 26, available at: