Hello Danny, I agree that does look like an interesting feature, I live quite close by and so have been up to look at it before now. It's bang on the line of the Roman Road (suspicious!)and is basically a rough circle of thorn trees, no sign of a ditch, but a slight banking under parts of the hedge. Tellingly, it doesn't appear on the 1847 OS 1:10,000, but does show clearly as a stand of trees on the 1890s OS 1:2500, so I'd suggest a date between those two. As to what it is, who knows - an artificial fox covert maybe? Farmer's dew ponds are often circular and sometimes partially surrounded by hedges, but this doesn't seem to have been one.
I like your blog by the way!