I was unable to find the site using the given GPS coordinates. I found it at 51.2850421 N 0.2711557 E [updated, thanks]. From inside the fort, go as far north as you are allowed to go; head east until you see a footpath down with a concrete marker. This is right along the edge of the public and private land. The site is then just to the left of the short flight of steps, a little way down the path.
Something is not right. This message is just to keep things from messing up down the road