The co ordinates take you to the  RHS  of the Memorial when facing the church

and the following equation is taken from there .

The first name  of the side you need to work from is Trelawney Badcock. RIP 

Sum of letters in 5th surname minus the sum of letters in 3rd surname = A

Sum of letters in 7 th surname  minus the sum of the letters in 8 th  Christian name = B

 Sum of letters in 9 th Christian name = C

Sum of letters in full name on 9th row  = D

Sum of letters in the 3rd Christian name = E

 Sum of 3rd surname = F 
 

The cache can be found at 

N 50° 50. A B C

W 004° 30. D E F. 
 

CHECKSUM OF ABCDEF = 23