"You need to fill in form IHT100 Inheritance Tax Account to tell HM Revenue & Customs (HMRC) when Inheritance Tax is due on a trust."
That appears to be saying that if no tax is due then you don't need to fill in a form. I hope that is true!
Hope is fine, but perhaps unfulfilled this time. Often comments on HMRC website whilst helpful are not typically exhaustive.
IHT100 may need to be completed whether IHT due or not.
The rules re IHT100 and discretionary settlements have changed over time.
The details are contained in SI 2002/1732 (for pre 6.4.07) and currently SI 2008/606 "Inheritance Tax (Delivery of Accounts) (Excepted Settlements) Regulations".