The County Durham Sites & Monuments Register, citing "ASUD, Durham University, A66(T) Sadberge Junction, walkover and geophysic survey, ASUD Report 813, June 2001" as its source, describes how this site was discovered:
"Geophysical survey in advance of a road scheme identified a circular enclosure containing at least one smaller circular feature. Outside of this a further circular feature and a number of other linear features were identified. The observed feature appear to represent the ditches of an enclosed settlement with round-houses and the remanats of a field system. The whole probably dates from the Iron-Age but may be earlier. The site fits into an established pattern of such small enclosed farmsteads noted from cropmarks across the area to the East of Darlington, indicating a settled densley populated area in the later prehistoric period."
Surveys of this sort are usually carried out because the monuments in question are about to be destroyed (ie. by the new road), and I wonder if any trace remains of this ancient settlement and enclosure. The map reference given here is accurate to within 1km.