If your local authority has awarded you less housing benefit than they should have, due to official error, they must make up the difference. An ‘official error’ means a mistake by the local authority, DWP or HMRC. There is no limit to the period for which arrears may be paid in these cases. However, if you were awarded less housing benefit because you failed to tell the authority something, see 14 above .
HB&CTB(D&A) Regs, reg 4(2)(a)