The cache is not at the published co-ordinates, but is less than a 15 minute walk away.
Complete the Lab Cache below where you will learn something of the Coaching Inns which served the Portsmouth-London and Chichester-Winchester coaching routes in the 18th-19th Century, and receive the values for A B C D E and a hint for the hide.
Petersfield's Coaches and Horses
Then enter the numbers you have into this formula:
N51 00.0AB W000 56.CDE