I have started to add the details of the chap and a few of his closest relatives onto my tree in a tentative way in the hope that it might throw up something from ThruLines. I know I shouldn't really do that but to be honest people shouldn't take things they find on others trees as gospel without checking it for themselves, should they? I do look at other peoples finding for clues but always check their findings for myself.
Anyway, the lady I found first who has the connection of 7cMs says that she has no direct relationship to my chap but that his brother was the 2nd husband of her great aunt. So I think that would explain why the link is so small, don't you think? I don't know what else I can do to try to prove that he is my great grandad, do you? Though I do think that it is quite likely.