The information board near the cannon suggests that it was fired to celebrate the investiture of Charles in 1969, but it definitely wasn't. I would have remembered the massive fuss - elf & safety, fire precautions, crowd control etc. (and I would have heard it) . It was probably created and used for the coronation and the investiture in 1911, but that was too long ago, even for me to remember.
Some of the cannon in the book are difficult to find and maybe not at the grid references given.