IHTM30455 | Determination of questions on previous view of the law: delay by HMRC
From HM Revenue & Customs · Inheritance Tax Manual
Where does not otherwise prevent calculations being revised in favour of HMRC__ after a change in practice, you should consider whether any undue delay by HMRC may have contributed to the case being settled after, rather than before, the change in practice. The instructions on reopening IHT liabilities () will be applicable. If it seems that this may be so, you should refer the case to your manager to decide whether in the circumstances we should treat as applying to prevent us from claiming additional tax.