That was the line I used to take roughly, but now there is a new checkpoint on the wall below Creigiau Hirion, here:

"After Control D the route takes an easterly line and a new pass-through control (FS) will be positioned at GR 684 624. From here follow the wall on the north side (south side is out of bounds) to checkpoint F at GR 694 624; this is unmanned, but you must dib your dibber."

I find that this line goes through quite rough terrain compared to the fast running trod slightly higher up.