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HMRC internal manual

Inheritance Tax Manual

Investigating accounts: failure to comply with request to deliver an account

If once you have identified the accountable person an account is not delivered after one reminder you should talk to Litigation to see if they wish to pursue the failure to deliver an account (IHTM36081) through the First-tier Tribunal. 

Alternatively Technical may advise you to issue a Notice of Determination (NOD) (IHTM37000) if there is enough information available about the estate or transfer to justify this approach.