The grid ref is SE 218 038. It is an odd looking beast and the cross is very smooth to the touch, almost polished. I can only think that it has been done with a powertool of some description quite recently, but possibly on top of an older design. The field walls date from the time of the enclosures and have obviously been built over the stone, plus quite a few others that lay along the side of the lane. Further down the path, there are a couple of other interesting stones at the junction of Slant Gate...
http://www.flickr.com/photos/andyhemingway/3465725526/in/photostream/
Plus an old guide stoop towards the top of the lane (again, built into the wall)...
http://www.flickr.com/photos/andyhemingway/3722842025/in/photostream
I'm sure that they are boundary markers of some description - but it all seems like a bit of an odd arrangement!