IHTM36091 | Failure to deliver an Instrument of Variation where additional tax payable: when can you seek a penalty?
From HM Revenue & Customs · Inheritance Tax Manual
You should consider seeking penalties under / where, following execution of an instrument of variation that results in additional tax, the ‘relevant persons’ () fail to meet the requirements of to deliver a copy of the instrument within 6 months after the day on which the instrument is made.
While also requires the relevant persons to notify us of the amount of additional tax that is due you should not seek a penalty where this requirement only is not met.