As I understand it, since it appears that the person has already died then X's share of the trust becomes part of their estate when they die.
It would be a very different matter if X died before the person who wrote the will. You mention in the OP "if any of them die before me" - that would be a very different.
My father's will had a similar condition. It then also had a further line that read:-
"If under this clause any share fails to take effect then such share is to pass to the part of the clause which does not fail and if more than one part does not fail then to those parts which do not fail proportionately"
Which basically says that if a beneficiary dies before the person then their share is distributed to all those other people who do benefit pro rata to whatever the will says.
If there is nothing in the will that says this then the gift to X simply fails and the amount passes into the residuary estate.
There should be further down in the will about what happens to the residuary estate. Something like "I give my Residuary Estate to ..." and this may be one person or a number of people.
There should also be a clause about the residuary estate if that were to fail as well:
"If under this clause any share fails such share shall return to my Residuary Estate and be distributed to the other beneficiaries pro rata according to their shares"
As I understand things, if any gifts in the residuary estate fail and there isn't this catch all sentence to redistribute the gift then it ends up following the intestacy rules.
Which is basically, the surviving spouse gets the first £322k and then splits everything above that 50% with any surviving children, or if they have all died then grandchildren.
But, since X is still alive then this doesn't apply.