May I suggest that you search via another direction and check what happened to Margaret Thelma Jones.
If you can find her and her William's Children, you MIGHT be able to check via the Kids what happened to William Henderson.
I am not a member of Find My Past however I did a check and found this, maybe it is Margarets birth.
If it is then a search of
https://www.centennialpark.org/memorial-search-list/harold-edgar-jones-58504/reveals that Harold Edgar Jones died 20.5.1987 and his wife Margaret 21.7.1969.
Should you locate a death/ funeral notice for Harold and or Margaret, you might be luckly and find a reference to Margaret Thelma. I am thinking that she may have remarried, then you could find her death notices, the kids names and continue the search that way.
Cass
Ps just found this in the Saville Index, there is a notice for Harold in the Adelaide Advertiser. If you email the SA Library, you might be lucky to get a copy of his notice,
https://savill.collections.slsa.sa.gov.au/#