Have you managed to find any additional information on the family. My grand mother was a Debnam who was born in South Africa on January 22, 1876 in a town called Thaba Nchu in the Orange Freestate. I am not able to find the information on her father Isaac Debnam born in 1836 (Some of our records show that he was born in Ireland with a last name Dednam) From the information that I have I would assume that Isaac Debnam (born 1836) was the son of either John Debnam born 1803 or Isaac Debnam born 1808.
I am also not able to get information on Isaac Debnam born 1781. I have found a link to an Isaac debnam in 1610 but cannot link the two.