If your cousin is correct, you are then a descendant of a gateway ancestor for the Magna Carta sureties. You might try the publications on Magna Carta descendants. There is also some information on WikiTree you might check out as well.
Since you have an extensive tree on WikiTree already, you could check the with the WikiTree relationship finder. However, be careful of taking these as correct, the “sources” for every generation should be verified. In my own case I have found some very dubious links.
It is easier to research from the present to the past, than from the past to the present.