I've never assumed its natural because of the stukeley sketch. Closer inspection tells me that this is not a natural feature as the shape of waden hill is gentle with a steep slope drop off but this mound is very round and unlike nothing else in the area.
Yesterday I went back to the pond and noticed a possible berm around the edge, of course this could be rubble that has been tipped over the top but there is no rubbish in the pond at all, not even a trolley!
PeteG