Intestacy - subsequently discovered minor beneficiaries

Hello. I am dealing with an estate where I was instructed on the basis of an intestacy, deceased was a bachelor, no children, no surviving parents, only one sibling. I was instructed that this was a close family, no requirement for family tree verification / involvement of genealogist etc. Grant of admin was applied for in name of sibling and estate progressed to point of distribution, just prior to which said sibling discovered that there may have in fact been another sibling or siblings…

The result of a family tree verification / genealogist search is two previously unknown full siblings, one of whom has died, leaving two minors to inherit in their place.

Had the minority interest been known about at the point of application for the Grant, two administrators would have applied.

Has anybody come across a similar situation previously? Will we be able to appoint a second trustee to hold the monies due to the minor beneficiaries or Will a new Grant be required?

Thanks in advance for any assistance.