Park at the suggested parking coordinates, and make your way to the posted coordinates. There you will find a black plaque. Take the lowest (or least) four digit number and call that X.
Do the following math: (X*0.49) + 9.011
Use your instincts to find a place closeby to implement your new number and find the geocache. Note that this location is not always open (see “related web page”). Please do not ask staff for assistance, and please avoid drawing too much muggle attention when retrieving, signing, and returning the geocache!