Meaning of Venwell or Venville

Will you allow me to make the following Query as to the custom of "Ven*well*" or "Ven*ville*"? Risdon, in his *Survey of Devon*, states it to be a right enjoyed by the tenants of land adjoining to Dartmoor of pasturage and cutting turf within the limits of the forest. He calls it "Fenfield, antiently Fengfield," but makes no allusion to the etymology of the word, or to the origin of the custom. Some of your correspondents can most probably afford information on both these points.