813.Paragraph 18 provides that, where the outcome of an appeal overturns an understanding of the law previously applied, with the effect that decisions in other cases are wrong, restrictions may be imposed on arrears which would otherwise fall to be paid. It also defines the terms used.
Sub->paragraph (1) identifies those cases where entitlement to benefit is restricted because an understanding of the law has been overturned by a decision on appeal (a “relevant determination”).  Sub-paragraph (2) identifies cases to which subsection (1) does not apply.
Sub-paragraph (3) provides that cases identified in accordance with sub-paragraph (1) above, are to be decided, as far as the period before the appeal decision is concerned, as though that relevant determination had not overturned the earlier understanding of the law.
Sub-paragraph (4) makes clear that appeal decisions of the courts, which determine that provisions in statutory instruments are themselves unlawful, are relevant determinations for the purpose of sub-paragraph (1)(a).  That is to say that, even though a provision in a statutory instrument is found to be ineffective by a decision on appeal, cases will be determined on the basis that it is effective so far as the period before that determination is concerned.
Sub-paragraph (5) provides that the restriction on the payment of arrears applies regardless of whether a claim or application for revision or supersession is made before or after the date of the determination of the appeal in the lead case.
Sub-paragraph (6) defines “the court” for purposes of this paragraph.
Sub-paragraph (7) clarifies that references to entitlement to benefit also covers (a) entitlement to any increase in the rate of benefit and (b) also that entitlement includes a benefit (or increase) at a particular rate.
Sub-paragraphs (8) and (9) allow regulations to be made to prescribe how the date of the relevant determination is to be determined.
Part III: Social Security Administration Housing Benefit and Council Tax Benefit Schedule 7 Paragraph 18: Restrictions on entitlement to benefit in certain cases of error