IHTM30201 | Payment by instalments: when instalments are payable
From HM Revenue & Customs · Inheritance Tax Manual
The facility provided by is for payment by ten equal yearly instalments. The first instalment is due:
in the case of a transfer on death, six months after the end of the month in which the death occurred,
in other cases, at the time the tax would be due if it were not being paid by instalments.