The field to the west of where you're standing holds the answer (It's also an SSI for the wildflowers that grow here in summer). As you walk towards the lane you'll cross 2 very shallow ditches in the field that were once 'leats' (small canals) that carried the water from the stream to the mill where it could be used to power a drop mill. It's not on the footpath but just past the old pumphouse (not associated with the mill or the leat's) you can see the remains of earthworks that would have dammed the stream into a mill pond.
This spot isn't only about History and Botany, though. It's the closest thing Ullenhall has to a Beach. Take some time to play in the stream, maybe build a dam.
You're looking for a microcache.