Hi, Natalie -
Looks like we do have a link. John Cadman King and his wife Martha Leonard were my great-grandparents. They married in Isleham 4 Jan 1873, and Isaac was their eldest son, born qe June 1873. At some point between then and 1876, the couple moved from Isleham to Fordham, where all their later children were born. I descend from their daughter Mary Jane, born in Fordham in 1885. Mary Jane married James Stephen Watson, and their son Cyril was my father.
I didn't follow up further on Isaac, but according to census returns, he and his wife Susan Gordon produced about 13 children all told, one of whom was a daughter named Elsie, b about 1905. (Another daughter, Gertie b 1903, was a friend as well as cousin of my eldest Aunt, and I remember her talking about "Uncle Isaac" and "Cousin Gertie" when I was little.)
John's middle name Cadman was his mother's maiden surname - which is common in Cambridgeshire, especially where the men's first names carry on through generations. The Cadmans were a big Isleham family, and I have traced them back to about 1740 in Isleham. The Kings were incomers from Chippenham.
Martha Leonard led me a merry dance for months because she was so hard to find. On her marriage to John Cadman King, she was named as Martha Leonard, daughter of Robert. She was actually the daughter of Robert Norman and Susan Harvey, born 3 months after the death of her father from cholera. Robert was the baseborn son of Alice Norman, who later married James Leonard, and it seems Robert was known by both surnames. It's possible he was actually the son of James Leonard, but there's no way of knowing that for sure. He was baptised as son of Alice, and married Susan Harvey as Robert Norman son of James, and in 1851 he and Susan and their baby son James are on the census as Norman. He was buried as Robert Norman. But when Susan registered Martha's birth, she registered her as Martha Leonard, daughter of Robert, and signed herself Susan Leonard. To confuse the issue further, Susan married James Ellington as Susan Norman after she was widowed, and by 1871, although her son James was still known as James Leonard, Martha was in service in Mildenhall using her stepfather's name. All very confusing!
However, I am confident that I did nail the right marriage partners - John Cadman King and Martha Leonard, so there's some solid ground for you to work back from. If I can be of further help, just shout!