Okay, thanks to some hints from you good folk, and a serendipitous connection with a distant rellie, I have a bit more info, so hoping someone might be able to add something...
Margaret Anderson came to Oz, aged 10, with her aunt and uncle William MALCOLM age 25 and wife Ann nee BLACK age 20. Ann was Margaret's mother's (Jane/Janet? BLACK) sister.
They arrived in Geelong in 1853 on the Childe Harold, along with Wm and Ann's daughter, also called Margaret (MALCOLM) who was 1 yo, apparently going to a Peter Malcolm, presumably a relative (?), at Batesford, near Geelong, Victoria.
Margaret ANDERSON was also living with the MALCOLMs in the 1851 Scotland census - they were living at Bothkennar, Stirlingshire. (Her age was '10' at that stage, too...)
Ann and Jane Black were the daughters of John BLACK and Margaret McEWAN (according to Ann's death cert which I now have). Ann was born in Alloa, Clackmannanshire, Scotland 1830-1832, year depending on passenger list and death certificate age.
So, perhaps, if Jane/Janet was an older sister the JAMES ANDERSON and JEAN BLACK Marriage: 10 FEB 1838 in Alloa, Clackmannan might just be be the one....!? (thanks, Monica!!) The rellie I've connected with has their marriage as 1838, but no more details.
Have discovered lots of information about sister Ann and her line, but poor Jane/Janet doesn't feature anywhere, so far.
Any further suggestions would be most welcome.