Corrective account on IHT100

I might be searching Tolleys incorrectly but can’t find an equivalent form for C4 for corrections to an IHT100 or the time frame for doing so. C4 is used to correct the IHT400 but does not apply to CLT.

I am preparing an exit charge for a trust set up in 2019 by a different firm and I think the nil rate band was incorrectly excluded in the original comp. There was a PET two year’s prior to the CLT which seems to have been used as the basis for no NRB being available - this PET is put on the IHT100 as a gift within box 2.3 which only relates to gifts where the transferor is no longer alive. The full nil rate band would have been used so additional tax paid is £65k.