It's a World War II observation post not Jackson's Barrow. I made the same mistake myself and couldn't believe such a fantastic barrow was not more widely known about.
However, Jacksons Barrow is actually the far less prominant bump a bit further up the hill which may according to some sources be a natural feature.
Ive been looking forward to visiting this one as its a bit of a curiosity.
A large 'horshoeshaped hengytype thingy' monument.
Its very well defined and has another smaller henge alligned with this one about 15 metres away.
Lots of lumps and bumps in this field along with some rather scary horny (not in the lusty way!) cows.