IHTM30435 | Reconsideration by taxpayers: decrease requested
From HM Revenue & Customs · Inheritance Tax Manual
Claims that the value of property should be decreased must be dealt with on their merits whether or not a statutory certificate of clearance (IHTM40010) has been issued.
There are detailed instructions on the statutory loss on sale relief for land (IHTM33010) and for shares (IHTM34011).