Proceed to the co ordinates where you will find 'Big Daddy' - a Victorian canon that was placed in it's location and abandoned, above which the searchlight post was built during World War II. The canon was discovered below the floor when the fortifications were excavated and exposed for tourists to admire. The way the hole has been left looks like the canon has fallen through the floor due to it's weight, hence the cache name.
To find the cache co ordinates, you need to count the main sections of Big Daddy, this number is A
The formula is.... N51 20.A(Ax3)A W003 06.87(Ax2)
Cache placed by kind permission of the Kenneth Allsop Memorial Trust who own and manage the island.