Symmetry N51 PA Bonus Cache

This is very tricky if you haven't read the book or solved the two grunt_futtock caches.

You only need one set of coordinates as it is a Symmetry cache and hence the numbers for North and West (after the N51 W000) are the same.

The clue is my hobbies, and in this case it is Harry Potter. Try looking up who taught Defence Against Dark Arts.