IHTM09242 | Ascertained values: when is a value `ascertained`?
From HM Revenue & Customs · Inheritance Tax Manual
A value is ‘ascertained’ when
we accept a value implicitly by accepting the tax on the value offered by the taxpayer: for example where the VOA provides an ‘as returned’ (IHTM23144) report, or we collect the tax on a valuable chattel without expressly agreeing the value, or
the taxpayer claims loss on sale of land relief (IHTM33001), when the substituted value becomes the ascertained value.