You will need to visit the close vicinity of the Post Office and the Clapper Bridge and answer a couple of very simple questions. The final stage is within 1km.
Near the Post Office you will find letters ABC and DEF - immediately above them are the 6 numbers they equate to. You do NOT need to enter the Post Office.
How many huge horizontal slabs are there on the clapper bridge? = G
How many vertical metal strips are there on the south side of the road bridge? = H
The cache can be found at N 50° 35.(G+1)BH W 003° 54.H(A-D)(D+E)
Check sum A to H = 37