This is a legend that first occurs (perhaps by the author's invention)
in the late-15th century "Chronicon Hollandiae comitum et episcoporum
Ultraiectensium" by the Carmelite Johannes a Leydis (aka Jan
Gerbrandszoon, Jan van Leyden and many other variants), on pp. 90-92 in
the edition by Frans Sweerts (Frankfurt, 1620).
According to this, Balduin was the second lord of Heusden, expelled from
Flanders by the Danes. He took service under King Elderic and his queen
in England, falling in love with their daughter Sophia. The couple fled
to his relative in Brabant, where she gave birth to sons named Eathmund
and Robert.
It is 100% proof nonsense.
Peter Stewart