I've sat on top of it in the rain. I think it is natural, it's an outlier from the Cotswold escarpment, connected to Cam Long Down. It's called Peaked Down on the OS map - close to Uley Bury hillfort. Rather like Picked Hill in Wiltshire in fact and I suspect the origin of the names are similar.

http://www.themodernantiquarian.com/site/5851/

It's quite a visible landmark from the M5.