IHTM14541 | Lifetime transfers: the charge to tax: grossing: when to gross-up
From HM Revenue & Customs · Inheritance Tax Manual
Grossing applies when the tax on the chargeable transfer is borne by the transferor. (IHTM14593)
You should gross up the value transferred if
the account or correspondence states that the transferor is to bear the tax
at the time of the transfer the transferor enters into a binding agreement to pay the tax
after the transfer, the transferor pays tax (even an instalment) direct to HMRC, or
the tax is paid from the transferor’s death estate (IHTM14593).
All these situations are caught by IHTA84/S227(1)(b). You should not allow instalments in cases where arrangements of this kind are revealed.