Failure vs determination of a trust

I think you need to publish the exact text that is intended to create a trust.
My guess is that the wording contains an "if then ...else" structure about a future event. This introduces an element of "wait and see". The future event is limited by a statute of limitations, so the result has to determine within a measurable time. [it is not good nor realistic for a society to have the dead to rule their descendants from the grave, especially if the descendants can be kept "in limbo" for years on end.].

The lawyer might have added this clause as a subtle get out from instructions given by a client trying to rule from the grave, that the client would not understand (and nor do I).

Alternatively it may have been written by a graduate of the circumlocution office:

http://www.bl.uk/learning/histcitizen/21cc/lang/control1/circumlocution1/circumlocution.html

Is the will trying to set up mini discretionary trusts for the benefit of individuals and their heirs, ie nil rate band discretionary trusts, each below the level where they will be regularly "milked" for InHeritance Tax.
[They would still be liable for CGT and Income Tax at the highest current rates]

I think part of tortuous language us because it is looking to a date in the future, by which time the shares, the beneficiaries and the law might have changed.

I think the word "share" is not referring to a certificate of ownership in a joint stock company but a subdivision of the estate as a whole?

Having read the will outline that a firm of solicitors appears to be offering to the members of a union (?), I think I realise it is written to keep the will legally valid and not partially intestate, regardless of what might happen after it is signed.
It just spells out that while the executors are controlling the estate it is a trust and it spells out what is to happen if a couple die at the same time [they don't the elder one dies first] by introducing a 28 day cooling off period.

http://en.wikipedia.org/wiki/Simultaneous_death

Is the will you are handling intended to name all the beneficiaries from the date of death and the intention is not to create long term trusts or even protect the survivors share of the family wealth against excessive care costs, dementia problems etc.?

So it is not a discretionary or interest in possession (life interest) trust ?
Was the will written before 2006 (taxation of trusts) or 2007 (transferable nil rate band)?
You might want to take advice on changing it using a deed of variation/family arrangement, if the effected beneficiaries agree it could be improved?

http://www.hmrc.gov.uk/cto/customerguide/page21.htm
http://www.hmrc.gov.uk/manuals/ihtmanual/ihtm16110.htm

Looks like HMRC are prepared to substitute a more generous 6 months (ie until the IHT is due)

IHTM16110 - Settled property: survivorship clauses in a will

Clauses in wills providing that the beneficiary will inherit only ‘if he survives me for 28 days etc.’ are very common.
IHTA84/S92 seeks to simplify the results of such clauses by providing that if the person survives then their benefit takes effect from the date of death. If they don’t survive then the interest of the person who takes the benefit also takes effect from the death.
The aim is to do away with any consideration of the trusts on which the property or its income is held during the waiting period.
The effect of S92 is that if under a will or settlement property is held for any person on condition that they survive a specified person for a period not exceeding six months, any trusts during the specified period are disregarded, and the dispositions which actually take effect at the end of the specified period are treated as having done so immediately on death.