HFMA Series is a loop around Harewood Forest and the caches are named after the part of the forest they are in/near. Each cache contains a snippet of either local history or wildlife. Although this cache is called Adders Corner, I have not personally seen any, but thats not to say they are not there!
When starting and finishing from GC5PWYN Sprat and Winkle No.1 the loop takes just over 2 hours to walk. Paths in the forest are mainly old concrete roads but some paths can be rutty tracks. Be prepared for nettles, brambles and mud! 
Most of the paths are fairly flat but some are on a steady incline/decline, (the hill at Park Farm is steepest (near GC5PX3P)).
After so many not founds I've checked and it's still there, so I've changed the hint, hopefully this will help;-)