IHTM30152 | Due date for payment: other charges arising on death
From HM Revenue & Customs · Inheritance Tax Manual
Other cases in which the tax is due six months after the end of the month in which the transferor’s death occurs include
Potentially exempt transfers (PETs) (IHTM04057) which prove to be chargeable transfers
tax charged under the gift with reservation (GWR) (IHTM04071) provisions treated either as part of the transfer on death, and so within IHTA84/S226 (1), or as failed PETs, and so within IHTA84/S226 (3A)
any extra tax payable on a chargeable transfer under Chapter III Part III IHTA 1984 (settlements without interests in possession) where the settlor dies within seven years.