To clarify the 'ac' question: 'Ac' seems to have been a Celtic suffix denoting 'place of' (rather like AS 'tun'). The placename's first element is almost always a personal name. In other regions of France it survives as "-at" in Auvergne, "-ach" or" -ig" in Alsace, "-é" in Poitou and Brittany, "-ey" in Champagne, and "-y" in the area south of Paris. I would guess that Welsh placenames ending '-og' are similar. IB