IHTM10813 | Transfers on death: use of estimated values
From HM Revenue & Customs · Inheritance Tax Manual
When completing the IHT400, personal representatives (IHTM05012) may include estimated values if they are unable to find out the exact value of any item in the estate. But before doing so they must have made the fullest enquiries that were reasonably practicable in the circumstances and when completing the account must include
a statement to that effect
a provisional estimate of the value of the property, and
an undertaking to deliver a further account of it as soon as its value is ascertained.
This is provided for in the declaration (IHTM10463) in the IHT400.