The Riddler is a fictional character, a comic book supervillain appearing in comic books published by DC Comics. He usually appears as an enemy of Batman. The Riddler is obsessed with riddles, puzzles, and word games. He delights in forewarning both Batman and the police of his capers by sending them complex clues. With this self-conscious use of a gimmick, the Riddler's crimes are flamboyant and ostentatious. The character is depicted as wearing a domino mask either with a green suit and bowler hat, or a green unitard with question mark prints. A black, green, or purple question mark serves as his visual motif.
The cache is located very close to the former RAF Watnall Bunker which is currently under rennovation. Just in the woods opposite GZ you'll find a marvel of military intrigue, a subterranean bunker used for Intelligence during the Second World War. I wouldn;t advise going to visit whilst it's a building site but if you have a look here: http://www.subbrit.org.uk/rsg/sites/w/watnall/ you'll find out some interesting history about where you are. Oh and your hint should you need it will require an internet connection (it's a picture hint on a link).
There have been some issues surrounding this cache going live but it was temporarily activated and in that time there was FTF by mrspinkrat so congratulations on that and hopefully no more issues and others can beat the Riddler. Enjoy!