SACM20020 | Appendix 1: Old Error or Mistake Relief: Essential Conditions
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.
There are a number of conditions which need to be satisfied before a claim under the error or mistake in a return provisions is accepted. Among the essential pre-requisites are