PSA5010 | Irregularities: Calculation errors
From HM Revenue & Customs · PAYE Settlement Agreements
Where you discover that there has been an error in the calculation of tax and NICs due on items included in a PSA you must
consider whether the tax and NICs paid in earlier years is also likely to have been incorrect
recalculate the liability for the year(s) in question, using estimates as appropriate
Where the error is minor, advise the employer to ensure that tax and NICs in future or enduring agreements is calculated correctly. See PSA2060
Where the error is substantial, resulting in an underpayment, consider Reg 110 determinations to recover the shortfall. See PSA4000