the split is currently at the end of the cycleway/footpath which is not far off where it should be. The split should probably be at the Surrey / Richmond boundary which according to an 1954 OS map is a few feet west of the footpath. Current Surrey maps shows the boundary at the western end of the Sunnyside Reservoir where there is also a sign on the fence saying Lower Hampton Road. Depending on which source is used it looks like the split will shift by a few 10's of feet. A more definitive answer would take a lot more digging & not sure it would add much more info. Going to mark as closed, reactivate if you wish it more accurate