This cache is part of the AAL (adverb adjective letterbox) series, which comprises random letterboxes in the Milton Keynes area, each with a randonly chosen adverb and a randomly chosen adjective. It is placed to mark Roundabout MK Events #13 (GC6THB4), which takes/took place nearby on 3rd November 2016. You're looking for a small clip-lock box wrapped in black duct tape.
THERE IS WATER VERY CLOSE TO THE CACHE. TAKE CARE WITH YOUR GEOKIDS.