Its simple! But I have been there a _lot_!
You come off the path to Kingston Russell when you go see the Grey Mare (btw see if you can find the colts, I've found one and I think the other is completely gone). Now retrace yr steps and carry on along the path. Don't leave it. Keep following it. Stop thinking you've missed it, keep going. There ya go. 10-15 mins past the Grey Mare and watch out for cow pats.