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HMRC internal manual

Self Assessment Claims Manual

Appendix 1: Old Error or Mistake Relief: Tax Must Have Been Paid

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.

It is a condition of error or mistake relief that the tax on the assessment which is alleged to be excessive must have been paid.

In practice relief may be given by way of discharge providing that the only reason preventing relief being given is that tax on the assessment is unpaid.