Vested or Contingent Gift of Share of Residue

I am instructed in the administration of an estate.

The Will divides the residue into shares, one of which is to be inherited by such of the deceased’s great-grandchildren as shall be living at the date of the deceased’s death or en ventre sa mere and if more than one in equal shares on attaining the age of 18 years.

There is then a generic giftover clause: if any of the persons named in paragraphs X above die before me leaving a child or children…

There is then a generic accruer clause: if at any time the trusts declared by those paragraphs above (including the giftover) fail, then they shall accrue to the others which have not at that time failed.

Were it not for the accruer clause, I would be minded to think that the gift was vested, but I am uncertain as to whether the accruer clause means that it is actually contingent.

I would be grateful for any thoughts anyone might have.

Thanks,
Helen