On the ancestry of the Grenville's and parents of Mauger, Count of Corbeil, their ancestor:
Richard I, Duke of Normandy (ggf through his eldest son of Robert II (The Devil), Duke of Normandy, who was in turn father by his mistress Harlette/Harlot of William I (The Conqueror); b 933; married 2nd his former mistress Gunnor and died 20 Nov 996, leaving [Mauger]. [Burke's Peerage, p. 1603]
---------------------------------------------
Mauger, Count of Corbeil, in right of his wife; married Germaine, daughter and heir of Albert, Count of Corbeil, and had [Hamon, Count of Corbeil], with an elder son (William, dsp a monk in the Abbey of St. Maur). [Burke's Peerage]
Note the above puts the descendancy through William in doubt. Of course being an elder son, he could have been Count at least for a while, and maybe had illegitimate children. Though they would not be Counts.