Any question answered...

Turn left at Jamm. Milkwood Road to Herne Hill.
Turn right off of Tulse Hill on Leigham Vale (it's a set of lights before the bridge after Tulse Hill station). Up LV. Turn right at the mini roundabout.

You can now either go up leigham court road for North of the common. Or straight down Valley Road for mid/south.

I'm stealing this advice and pretending it was my own as it's better than mine