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HMRC internal manual

Inheritance Tax Manual

Land valuation cases: action where there is no appeal

Unless the taxpayer appeals (IHTM37013) against it within 30 days after the Notice of Determination (NOD) was served, the determination will generally be conclusive, although late appeals may be entertained in accordance with IHTA1984/S223. Any application for a late appeal to be considered must be referred immediately to Litigation.

If no appeal is received by the next review date, you should ask your manager for advice. They will probably tell you to raise a calculation on the value determined.