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HMRC internal manual

Inheritance Tax Manual

Published guidelines: form of an instrument

The only stipulation in IHTA84/S142 about the form of an instrument is that it must be in writing. It does not have to be a formal Deed or even writing that is signed as a Deed. You can accept a letter or note from the beneficiary redirecting their inheritance as a valid variation so long as the document conforms to the guidelines and otherwise meets the conditions of IHTA84/S142.