Frederick Lewis Weis's Ancestral Roots of Certain American Colonists Who Came to America Before 1700 identifies Robert Beauchamp as III (instead of IV) and indicates that his wife might have been Juliana Brett.
More recent research by Douglas Richardson, available at the soc.genealogy.medieval archive, indicates that she was Juliana Dourton, whose maternal grandmother was the mistress of John Marshal, elder brother of the famous William Marshal.
WikiTree profile Brett-78 created through the import of TheOlivers_2011-06-11_01.ged on Jul 9, 2011 by Rich Oliver. See the Changes page for the details of edits by Rich and others.
This person was created through the import of Acrossthepond.ged on 21 February 2011. The following data was included in the gedcom. You may wish to edit it for readability.
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Featured National Park champion connections: Juliana is 22 degrees from Theodore Roosevelt, 27 degrees from Stephanus Johannes Paulus Kruger, 21 degrees from George Catlin, 22 degrees from Marjory Douglas, 30 degrees from Sueko Embrey, 22 degrees from George Grinnell, 28 degrees from Anton Kröller, 20 degrees from Stephen Mather, 27 degrees from Kara McKean, 24 degrees from John Muir, 20 degrees from Victoria Hanover and 32 degrees from Charles Young on our single family tree. Login to find your connection.
'Is there any connection between Alice de Colville and Pain de Dorton? Again I can't answer that question but my guess is not. Presumably the Marshal family settled the manors of Dorton and Maidencourt on Alice de Colville, and they in turn passed by inheritance to her daughter, Sibyl de Bermingham, and grandson, Robert de Beauchamp. If Alice only had illegitimate issue, however, technically they could not inherit from her. However, if a settlement of Alice's lands was made on her daughters at the time of their marriage, I assume the king would have honored that settlement.' https://soc.genealogy.medieval.narkive.com/QEYFCkjE/juliana-brett