This is part of Maud Heath's Causeway.
Maud Heath's Causeway is a pathway dating from the 15th century in rural Wiltshire, England. On both sides of its crossing of the River Avon. the path rises above the floodplain on sixty-four brick arches (built 1812, largely reconstructed in the 20th century)] alongside an undistinguished country road between Bremhill & Langley Burrell
The causeway is the gift of the eponymous Maud Heath, who made her living carrying eggs to market at Chippenham. She was a widow and childless, and when she died she left money to improve and maintain the path along which she had tramped to market several times a week for most of her life. Five hundred and some years later, the charity still maintains the path out of her bequest.
At the posted coords you will find a rectangular sign with a single word on it. Let this be ABCDEFGHJ (no I)
The cache can be found at
N51 (3A)-B.(C/(E+G)) (H-F) (D+G)-B
W002 (E-G) (F-J).00E