Well Anns - it looks like I can safely eliminate one John Hall.
Unfortunately the one I can eliminate is the one you thought was your connection.
1851 Census
Treassowe, Ludgvan
John HALL, head, mar, 35, tin miner, Ludgvan
Maryann do., wife, mar, 35, miners wife, Ludgvan
John do., son, 9, scholar, Ludgvan
Mary Jane do., daur, 7, scholar, Ludgvan
Zacharias do., son, 5, scholar, Ludgvan
John son of John and Mary Anne Hall of
Treassowe baptised 23rd January 1842 Ludgvan (born 15th December 1841)
Mary Jane daughter of John and Maryanne Hall of Treassowe baptised 14th January 1844 Ludgvan
Zacharias William son of John and Mary Ann Hall of Treassowe baptised 12th April 1846 Ludgvan
Ann Maria daughter of John and Mary Ann Hall of Treassowe baptised 14th January 1849 Ludgvan
Ann Maria daughter of John and Mary Ann Hall of Treassowe baptised 9th November 1851 Ludgvan
Philip Henry son of John and Mary Ann Hall of Treassowe baptised 5th June 1853 Ludgvan
1861 Census
Crowlas, Ludgvan
Mary Ann HALL, head, widow, 45, domestice, Ludgvan
John do., son, unm., 19, tin dresser, Ludgvan
Mary Jane do., daur, unm, 17, stamps girl, Ludgvan
Zacharias do., son, unm, 15, tin miner, Ludgvan
1871 Census
Wheal Baddern, Kea
John HALL, head, mar, 29, Agent, Wh. Jane, Kea,
Treasowe LudgvanMargaret do., wife, mar, 30, Agent's wife, Lower Quarter Ludgvan
Mary E. do., daur, 1, Kea
John do., son, 5 mo, Kea
John HALL married Margaret CHAMPION in the December Qtr of 1868 Penzance Reg. Dist.
As you can see from the baptisms of the children above they were all resident at Treassowe, Ludgvan when baptised. By 1861 John senior has died and son John is still living at Ludgvan with his mother and two surviving siblings.
In 1871 John Hall age 29 was married and living at Kea. But the most important thing here is that he has stated his birthplace as
Treassowe, Ludgvan.
By this time your John Hall had been in Australia for at least six years.
Next step now is to see if we can track the movements of the John Hall born at Gulval to see if he might be the man you are looking for.
CT