HMRC says that the trustees should: “Use this form to tell us that a beneficiary’s qualifying interest in possession in a trust has come to an end due to their death and Inheritance Tax is due”.
Presumably, it would be worth sending a covering letter to HMRC to explain the lack of administrator (or ask the beneficiaries to apply for a letter of administration). Hopefully, HMRC will then write back and set out how much tax is due. But I have no experience of this.
One thought is that if the estate is an intestacy with no, or very few, assets then it will be spouse exempt. That being true you can calculate the tax (400,000-325,000)@40% (or whatever other adjustments might apply) and offer to settle with HMRC on that basis. Probably more acceptable to your beneficiaries than trying to force the widow(er) to take any action??