In reply to UKH News:
Is Merddyn making an assumption here: that the highest point of the hill necessarily lies on the watershed? He's established that, so far as can be determined, the Wales/England boundary follows the watershed. However, a hill summit can lie away from its main watershed. As an example, Lochnagar forms the watershed between the upper Dee and Loch Muick. Cac Carn Mor (1145m) lies on that watershed; but the highest point, Cac Carn Beag (1156m), drains entirely into the upper Dee.