The coordinates given above will not take you to the cache. However, the magnetic cache may be found by gathering information included in the Knockholt Pound Adventure Lab Cache.
Knockholt Pound has some historic sites and buildings. This cache is the Bonus Cache linked to the Knockholt Pound Adventure Lab Cache. Using the answers to the Adventure Lab Cache questions, the cache may be found at:
N51 18.(A+C)(A-B)A E00 07.(B+C)(A+C)(B+D)