Marriages can be misleading as they often take place in the bride's parish.
Sometimes the register will say if one party is an outsider. But sometimes they lied.
I don't see any Sheppards in Little Fransham, but there's evidence of a Ward family. A William Ward married Sarah Reeve there in 1800. They had a Harriet, who died, and a Thomas, and another Harriet, with big gaps. They could well have had other kids who weren't registered.
The 2nd Harriet married there, suggesting that they lived there the whole time.
A David Ward married a Powley there in 1824, but he could be a stray as his bride was local, there were loads of Powleys. However, he stayed around to have kids, described as a husbandman.
Could be impossible to prove or disprove that John belonged to that family.