Difficulty for this one is based on fact that you have to solve intro, 1, 2 and 3 in this series to get to this one. Each of the geochecks for those caches has a question in it, and the answers are A,B,C and D.
Assume N51 and W0 as usual. Then the cache is at
(A1+A2+B1+B2+C1+C2-D1).B2 (C1+C2+D1) (A1+C1)
(A1+B1+B2+C2).(A1+D1) (B1-B2) D1
Again, the GPS was being naff, but what you are looking for is just off the path.