The following taken from Jennie Lee Cobban's (of Barnet & District Local History Society) book 'Geoffrey de Mandeville and London's Camelot':

"This ditched enclosure, whose date and function are unknown, covers an area of ten acres...in 1913...when Mr Taylor and Mr Gillard recognised the 'camp', as something unusual, they invited a Mr Reginald Smith of the British Museum to inspect it in August 1917. He confirmed that the ditched enclosure was indeed a man-made feature. A little later on, the discoverers spotted several mounds, or tumuli, near the centre of the camp...In 1952, Derek Renn of the Society of Antiquaries investigated the camp and his report suggested that [it] might be dated to Iron Age times...He came to this tentative conclusion by comparing it with Loughton Camp in Epping Forest, which is very similar to our site in both design and situation...Hendon and District Archaeological Society...in the 1980s...surveyed and plotted the site thoroughly and noted that the camp's original perimeter was approximately 800m and that around 60% of the bank remained at the time, along with traces of the ditch. (The bank and ditch surround a clay feature known as Newman's Hill.)...HADAS noted that the site seemed far less well defined than in the 1950s. Erosion had taken its toll."

I can confirm the latter, with the bank being at most 4 foot high, and have found no trace of any tumuli (although the area is covered in trees and brambles so may conceal something.

MS Cobban also writes:

"A Late Bronze Age socketed knife ...came to light...on Hadley Common...Various prehistoric flints have been collected in the Hadley area over the years..."

...but not Iron Age. Hmm, so perhaps I should have typed a possible Iron Age site!

She goes on to speculate that the enclosure may have been a fortification of some kind, or had a religious function, or that it may have been a cattle enclosure.

[Articles & reports on these various investigations are held at Barnet Museum, Wood St.]