These photographs show the Long Newton reservoir being built by Tees Valley Water Board between 1901-1905, concluding with an opening ceremony on 20th September 1905. Courtesy of Jim Prentice. 
Picture Stockton Archive
A Pictorial History of Stockton-on-Tees
I lived at Long Newton Reservoir as a child in the 1970’s. We had great fun climbing in all the concrete pipes stored there on the hill. The reservoir is divided into two halves, so it is not always full. It was pretty scary walking along the concrete path between the two halves. Great fun when one half was empty and we took our bikes into the reservoir. We often spent the whole day up there, avoiding the workers at the pumping station who might tell us off, pic-nicing with other kids from the next door farm and village. This, in the days where we were allowed to climb trees, disappear for the whole day, cycle along the A66….the fun we had 🙂
I stand corrected. There were rumors that underneath the reservoir there was a nuclear proof military command bunker.
The resevoir is equidistant between Sadberge and Longnewton (check Google maps)& officially is Longnewton Resevoir. It is raised up, so not easy to spot from the road. Several years ago, during one of those periodic water shortages, water was taken from here by tanker to the parched south.
I lived in Long Newton and dont recall a reservoir, I remember one at Sadberge.