IHTM09761 | Sales of related property, etc: sales of shares in companies
From HM Revenue & Customs · Inheritance Tax Manual
If the property for which relief is claimed consists of shares in a close company (IHTM14851) you should refer the claim to SAV for advice. They are responsible for the application of the relief in these circumstances and will also consider whether the sale satisfies the additional condition in IHTA84/S176 (5).