Jenn, not sure if she did look further on the electoral roles.
Neil, yes the grandfather died in NSW. Have his death certificate and probate.
Sue, I hadnt thought of FB. Again, glaringly obvious once its been suggested. This is why I like this site so much, people suggest stuff you have not thought of.
I am not totally comfortable with putting my grandfathers name or his last wife's name here. I have so much to go on now, that hopefully I or my father can find his half sister. If we dont, then I might come back with their names.
Thanks for all your thoughts.