Not my line, but I can probably give you a little bit of info.
Stephen's father John, was the son of John and Cordelia. Baptised 3 March 1845 in St juliot.
John Jose and Cordelia BOX were married at Otterham on 1 June 1837. John noted as a resident of St juliot, Cordelia of South Petherwin and a sojourner there.
There are a couple of Johns who are of the right sort of age to be the chap who marries Cordelia, baptised in St Juliot area -
John, son of William and Grace, 1806
and
John, son of Digory and Jane, 1809
Any use to you?