Cornwall, like Wales, specialises in recondite saints hardly known outside the area. Crowan's church is dedicated to St. Crewenna. The parish church of Sennen (close by Land's End) is dedicated to St Senanus / Senana / Sinninus (take your pick as to preferred version) -- we are told that the church "has also been dedicated to John the Baptist".
I've been accustomed to reckoning the "By Tre, Pol and Pen" bit, as coming from the local ballad "The Gate Song of Stowe": of Civil War origin, telling of the doings in that conflict of Sir Bevil Grenville, fighter on the Royalist side -- Sir B. having been reckoned either a knight in shining armour, or a fiend in human shape: depending on one's allegiance in that historical episode.