I have published the hagiographical "dossier" of saint Melar in 1997 as a thesis (EPHE, Paris-Sorbonne). I tried to show this "dossier" is mainly formed with 2 vitae of the saint, the oldest composed in the late 11th century, the second one at the beginning of the 13th century. As Phyllis said, there is a text which adapts the story to Cornwall : this text is found in the Legenda of John of Tynemouth (mid 14th century), and has been composed (may be by John himsel) for the use of Amesbury Abbey where some relics of a saint bearing the same were kept.