The Secret of Platform 13 is a children’s novel by Eva Ibbotson, published in 1994 (three years before a certain other book you may be thinking of).
In the story, a gump is a hidden door which opens every so often to reveal a tunnel into a totally different world. The one at King’s Cross opens for exactly nine days every nine years. It is under platform thirteen, and the secret door is behind the wall of the Gentleman’s Cloakroom. The gump leads by a long, misty tunnel to a secret cove where a ship waits to take those that wish it to an island so beautiful it takes the breath away. When it opens, four mysterious figures come through into the streets of London. A wizard, an ogre, a fey and a young hag have come to find the prince of their kingdom, stolen as a baby nine years before.
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King’s Cross Station has never officially had more than 12 platforms, so you won’t see any labelling for this secret one. To find the cache, go past the people who are queueing for the wrong magical platform, and head to the toilets. There is a mysterious locked door next to the ‘Loo of the Year’ awards. I suspect this might be the way to Platform 13. There is a 2-digit number on the door, preceded by the letter ‘G’ (standing for ‘Gump’ of course). Let that number be AB, and use it to work out the final coordinates of the cache, which are not too far away.
N 51° 31.((2*A)+B)(B-A)((2*B)-A)
W 0° 07.(A+B)(B-(2*A))(3*A)
Checksum for all digits: 51
Final location is available 24/7, but the station may be closed if you visit very late at night. I wouldn't recommend visiting this area late at night anyway.
If the Gump happens to be open when you visit, please do let us know before you head off to the Island, so we can come too!
This cache is part of my ‘Fictional London’ series, check this list to see them all (more to come).