How do we establish Adam as a child of John FitzGeoffrey and Isabel le Bigod?

Hi folks,

I'm triying to figure out why Adam FitzJohn is listed as a child of John FitzGeoffrey and Isabel le Bigod. Yes, obviously FitzJohn means his dad was named John, but it doesn't mean his dad was *this* particular John, there being more than one guy named John in the British Isles at the time. :)

I can find corroborating genealogies on the web that go back as far as Adam... and I can find corroborating genealogies on the web that go forward (from, say, William the Conqueror) as far as Isabel le Bigod... but I haven't been able to find any genealogy on the web *so far* that corroborates Isabel being Adam's mom. I can find plenty that give a list of kids for Isabel that does *not* include Adam.

So... who knows more than me, and has a URL? :)

Thanks!