This is a muggle-heavy area, right at GZ, which might prevent your access to the cache. If you can children of a run-and-in-forests-and-climb-up-and-down-muddy-banks age, they would make the ideal smokescreen/cache retrievers.
As a further complication, GPS is surprisingly tricky in this immediate area, so I'll tell you now: GZ is a bomb hole with either a rope swing or a rope (depending on how recently it's been fixed/broken) dangling from an overhanging branch in the centre.
Well done to 01582 for the FTF.