The cache is not at the above coordinates but if you did enter the farmers field you would find an underground WW2 bunker! However, it would not help you to solve the puzzle.
Jack Tar was a name given to describe anyone serving in the Royal or Merchant Navy during the time of the British Empire. It was such a common title that it appeared in several poems and songs.
In my opinion two of our greatest sailors have been Captain Cook and Lord Nelson. If you were to add the two together you would not only get the greatest ever Jack Tar but you would also find the cache!
You can validate your puzzle solution with certitude.
Ybpngvba: frr pregvghqr.