Gromit is a little confused as it would appear that this isn't the Wallace he was looking for.
The story goes that whilst on their way to the Battle of Falkirk William Wallace and his followers made camp in this area. In preparation for the battle it is said that a large upright stone was used by Wallace to sharpen his twa edged claythorn.
This is apparently the very stone used by the legendary warrior.
There is some debate about this but I'll leave that up to you to decide.
There's a place to pull in at the cache if travelling by car, there's no need to enter the field as the cache is accessible from the verge.