No explanation of what the connection between the names is or how someone might help?
John Hunter St Ledger would of course be the son of
William Henry St Ledger
Ellen Hughina R Hunter
who married in 1896 in Birkenhead.
If William was born in Falmouth in 1866, he was the son of James, a mariner, and Mary, on the 1871 census in Falmouth; his surname has been multiply corrected at Ancestry by two users who would likely be the people to ask about him.
Ah, there we are. The name Mary is a mistranscription and has been corrected by the same people at Ancestry to Fanny. They are the couple I referred to earlier:
James St Ledger
Fanny Pascoe
who married in Falmouth in 1863.
So William Henry St Ledger's wife was Ellen Hunter, but his mother was Fanny Pascoe.
In 1901, William, Ellen, John, 3, and Christiana, 1, are in Gravesend, where William is the chief mate on HM customs cutter. (Your saying "at" Gravesend was what made me think first of the house in Torpoint.) In 1911 John is at private school in Gravesend, Ellen is with her sister in Birkenhead, and William, I don't see.