Richard de Grey

Process of elimination says he was not the son of Robert (no children at all), Anketil (one son John, one granddaughter Eve - end of the first Rotherfield line), or Thomas (one daughter, Alice). The monk was out of the running from the get-go. That leaves William.

It is highly unlikely that he married an "Eva de Reviers" - that's the bloodline of the Earls of Devon, and the Greys were not yet flying that high.