About Thurstan de Bastenburg
There is a mention of this name in the MEDLANDS database:
GIROIE [Geroius], son of ARNAUD "le Gros" & his wife --- (-1050 or after). ....
m GISLA, daughter of THURSTAN de Bastembourg & his wife ---. Guillaume de Jumièges records the marriage of Giroie and Gisela daughter of "Toustain de Montfort" and names their seven sons and four daughters, in order, "Ernauld, Foulques qui périt avec le comte Gilbert, Guillaume, Raoul Male-Couronne, Robert, Hugues et Giroie et les filles Heremburge, Emma, Adelaide et Hadvise". Orderic Vitalis names her and her father, as well as her seven sons and four daughters.
No evidence found for his being of Norwegian descent.