I had previously noticed that the public toilet on The Strand is "twinned" with another water closet in the developing world. Then, this year, I realised that the WC at Porthcressa is one of a different set of twins. Finally, we used some teamwork to establish that there are at least four sets of twinned water closets on St Mary's.
What made this an obvious candidate for a puzzle cache is that each twin has plaque with a photo and latitude and longitude displayed in situ. (I've found a blurry photo of one online but I think you need to visit the physical locations to get the required information.) You, and a friend whom you can convice to help out, need to use the four checkers below to gradually reveal the coordinates of the physical cache.
Looking on a satellite image, three of the locations look spot on but the "Afghanistan" one looks odd. It is actually in Tajikistan, but very close to border and must be within a few miles of the genuine location. To clarify, you should enter the coordinates on the notice.
Afghanistan:
You can validate your puzzle solution with certitude.
Democratic Republic of Congo: Missing. Answer to input = S 01 41.743 E 028 59.670
You can validate your puzzle solution with certitude.
Guatemala:
You can validate your puzzle solution with certitude.
Nepal:
You can validate your puzzle solution with certitude.
We suggest that, if you are staying on St Mary's, you visit the WCs before you travel to the off-islands.
The cache is not hidden in a wall.
Maintenance: One member or other of our extended family will be lucky enough to be staying on the islands for more than half of the year and will keep an eye on the cache.