SACM20025 | Appendix 1: Old Error or Mistake Relief: What it Covers
From HM Revenue & Customs · Self Assessment Claims Manual
From 1 April 2010, it is not possible to make a claim for the old error or mistake relief for any period. The relief is replaced by overpayment relief, see SACM12000+. You will only need the guidance in SACM Appendix 1 if you are dealing with a claim made before 1 April 2010.
The expression `error or mistake’ covers
errors of omission such as the non-deduction of an admissible expense
errors of commission, such as computational or arithmetical errors
errors arising from a misunderstanding of the law, and
erroneous statements of fact.
If the taxpayer contends that they have not been given credit for an amount of tax deducted at source, it is not necessary for them to make a claim under Section 33 as they have an automatic entitlement to the tax deducted.