British Museum stuff 

I've removed:"In the British Museum there is a genealogy in a manuscript of the Cottonian Collection showing a lineage of Gruffydd ap Cynan. It shows he is the father of Owain Gwynedd and the grandfather to Madawc (another spelling for Madoc). Also in the same collection is a Latin manuscript Vita Griffini Regis (The life of King Gruffydd) that says Gruffydd is the son of Cynan ab Iago and father to Owain Gwynedd - and Owain's son found unknown lands." Earlier I removed "This manuscript as talked about in the Cottonian Collection has a date on it of 1477". 
First, there are quite a few Madocs/Madogs/Madawcs etc, so genealogies are of not much use here.
Secondly, why are we claiming that the manuscript says this without a link to it? There is an annotation that says " Vita Griffini, filii Conani, regis Venedotiae vel Northwalliae, à Thelwello jurisperito Latine versa: ad finem carmina reperiuntur Meredithi de Madoco, filio Oweni Gwynnedd, et ejus navigatione ad terras incognitas; Wallice. Vixit Meredithus iste circa An. 1477". Note that this is not a date on the manuscript. We need a reliable source that has actually read the manuscript as the annotation fails as an RD (as clearly does Deacon. Until a reliable source reads  and comments on anything in it that might have to do with Madoc, this doesn't belong here.