I’d failed to find this site on a couple of previous occasions, and had been told it’s tricky to find, but strangely went straight to the stone this time round with no problems whatsoever.
From the A229 southbound, take the slip road immediately after the Shell garage, and park. Walk back toward the garage, and follow the path round to the right. There is a bridge across the railway line, the stone is less than 100 yards up the footpath on the left.