This is the Purton Ships Graveyard Lab Bonus Cache, you need to complete the nearby adventure lab to get the coordinates for this cache. The coordinates are given to you after completing stage 5 of the adventure lab.
I tried to place the bonus cache closer, however much of the surround land is owned by the Berkeley Estate and a couple of other spots were not muggle proof! You can cache and dash as there is parking near the cache. However the walk to the cache provides lovely views of the Severn and you get to see Purton (and beyond) from a slightly different viewpoint. Good luck!ย 