arthur lois henderson married margaret lucy cook, 9th may 1931
father ernest albert henderson,
margarets parents were
frederick cook(either uncle or great uncle of mine) and margaret doody
Deidre the index shows the marriage quite differently...perhaps have another look at your records

Marriage in SA
HENDERSON Ernest Albert 23 years Status Single Father Arthur HENDERSON
COOK Margaret Lucy 18 years Status Single Father Frederick William COOK
9 May 1931
At St Laurences Priory North Adelaide Ade S 327/406
Birth in Victoria
HENDERSON Ernest Albert
Father Arthur James Mother Mabel Mary Ann GREER
At Williamstown 1907 Reg#31202
Marriage of parents in Victoria
HENDERSON Arthur James born Kensington
GREER Mabel Mary Ann born Williamstown
1907 Reg#4240
Cheers
Cando