So... unfortunately the mystery is not yet solved.
This
Horace Warner's mother was Maria Batchelor, not Maria Postlethwaite...
have just revisited Horace's death cert issued in Victoria. His parents are clearly Richard and Maria Postlethwaite
seems to indicate that my grandfather's original surname was definitely Postlethwaite. How certain is this though? By that I mean, who would have provided the information for the death certificate, and how did they get their information?
The existence of a "Richard Warner and Maria Warner (nee Postlethwaite)" and a different couple "Richard and Maria Postlethwaite" would seem to be too much of a coincidence, and would suggest that they are the same people and that my grandfather, for whatever reason, thought of himself as more of a Postlethwaite than a Warner, and said so at the end of his life. On the other hand, he certainly did not stay in London to become a well known photographer during the 1900's.
Curiouser and Curiouser.