IHTM30151 | Due date for payment: death transfers
From HM Revenue & Customs · Inheritance Tax Manual
contains two rules which apply to transfers on death:
under the tax is due six months after the end of the month in which the death occurs
and
under personal representatives () must pay all the tax for which they are liable on delivery of the IHT400. However to the extent that the instalment option is available this requirement is limited to payment of any instalments which have then become due.