Stuart, that is a surveyor's benchmark, not a WD boundary marker. The WD symbol doesn't have the line across the top. Looking at an old 1960s map (attached) you can see it shown as an arrow against the bank's wall and the legend "BM 123.94" gives the precise height of the deepest part of the horizontal cut. Incidentally, the cross and "121" is a spot height that isn't marked but just added to make understanding the terrain easier.