I would be grateful for help with what to do in the circumstances of this case in its early stages.
I understand that it is not possible to dispose of a spes successionis. Nevertheless, how does a beneficiary named in a Will where the testator has not died, effectively alienate his interest under the Will, the particular circumstances being that T has lost mental capacity and the stepchildren under his second marriage, (second wife has died) wish to disclaim the benefit that they will take under the Will as it stands, such that it accrues to the children of the first marriage.
We don’t know yet, not having seen the Will, whether the stepchildren have pecuniary legacies and the children receive residue, but we think so, and if correct, we wonder if the way forward is for the stepchildren to purport to disclaim, and in addition (1) covenant with the children to disclaim and (2) irrevocably appoint the children their attorneys for the special purpose of disclaiming in their name if they, the stepchildren, do not comply with their covenant and (3) covenant to indemnify the children, making this a condition, breach of which would entitle the children to liquidated damages equal to the amount of the stepchildren’s benefit under the Will. There is goodwill between children and stepchildren at the moment, but that could change of course and so the children want it made enforceable now, the only alternative appearing to be an application for a statutory Will supported by all children. The testator is assumed not to have sufficient capacity to make a codicil or new Will.
I hope that members of the forum will have met this and be able to assist. If going my way as above, I’d also want to decide whether to separate the disclaimer, made even if not enforceable, and recite it in a separate deed comprising the covenant, power of attorney and indemnity.
A pre-death disclaimer might be difficult. Smith v. Smith  1 W.L.R. 1937 suggests that a disclaimer requires something to bite on, so that in the case of a will, the testator must have died.
But Re Lind  2 Ch. 345 recognised the validity of a mortgage of a spes successionis in relation to the estate of the mortgagor’s mother, which survived the mortgagor’s bankruptcy and was enforceable against the estate on the mother’s death. In re Clarke (1887) 36 Ch.D. 348 recognised the validity of an assignment in so far as it related to a future interest in an estate.
So perhaps an assignment is the way to go.
I am much obliged to Josh Lewison for his very helpful comments. The one thing I did not think was likely to work was assignment!
My understanding is that property as yet not acquired (often referred to as after-acquired or future property) as is the position here as the testator has not died, can be the subject of an assignment. Future property would include an expectation of benefiting under a will Hobson v Trevor (1723); Collyer v Isaacs (1881); Re Ellenborough .
A disclaimer would not be possible as there is nothing to disclaim. A disclaimer is simply non-acceptance of a gift and thus at the time of making the disclaimer the disclaiming person must be in a position to accept a gift. In the case of a possible benefit under a will until the testator dies there is nothing for a potential beneficiary to disclaim.
This intriguing thread feeds into the IHT issues adverted to in Challen v Challen and Another  EWHC 1330 (Ch) (27 May 2020). The judge decided that the Forfeiture Act operated but disapplied it. His view was that the claim was in time notwithstanding that nearly 8 years had elapsed between original conviction and its being quashed.
The property and IHT issues seem to be:
1 the entitlement to the deceased’s estate in the events which had actually occurred and the effect upon that of the decision. The effect of a declaratory judgment must apparently be that the claimant never had an entitlement but now does.
2 The claimant intimated to the court that she had no intention of disturbing the historic destination of the estate. The court could only speculate whether she might have already done so effectively.
So did she at any time have a spes successionis and what if anything might she have done to dispose of it before judgment (doubtless with a huge discount for the contingency) and if she did nothing of the sort is she now at risk of omitting to exercise a right (to pursue her entitlement)?
And is some poor adviser at risk of being accused of not having anticipated all this at the optimum moment (by customary judicial hindsight) at least if it now enters into the murky panoply of those circumstances requiring his putative topical state of the art expertise to be pro-actively exercised?How far is a criminal specialist to be second-guessed by supervening judicial omniscience requiring letters to be written promptly to insurers and will it matter ultimately whether he or she has a property specialist in house?
The case seems to make these issues more concrete, if unusual, than just the type of undergraduate’s exam question I recall well e.g. " The Law of Tort. Discuss".
I should have added that the claimant is apparently keen (per her lawyers in The Times) to recover the inheritance tax paid presumably on the footing that the spouse exemption now applies. As HMRC never tire of pointing out there is no time limit for IHT outside of a clearance certificate and (sed quaere) its M. Mouse letter lookalike.
Thank you, Malcolm and Jack, for further enlightenment. I greatly appreciate this. We think that we know the way forward now. The forum has worked well for me.