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HMRC internal manual

Inheritance Tax Manual

Negligence: miscalculation

It may be negligent to submit an account which included an obvious arithmetical error in a schedule, or which was incorrectly extracted from a document. Although a penalty may be due it is likely to be substantially abated (IHTM36204).

There might be different considerations if we found that a professional personal representative was frequently making such errors. It could well be thought that she or he was making deliberate mistakes in the hope that they would not be noticed. In such a case you would need to consider the penalty abatement very carefully.