This is the back corner of The Salvation Army's Sunbury Court Conference Centre. The Salvation Army has operated conferences year-round on this property for many years. Since 2014, Sunbury Court has also been the home of The International College for Officers (ICO).
From this location you can almost see over the fence into the sprawling property which The Salvation Army purchased in 1921. At that time, the property, and particularly the mansion (originally built in 1723 by John Witt), was dilapidated and had been neglected for many years. The Salvation Army was able to purchase the site at an incredibly discounted price. In order to restore the mansion and bring the property back to its original beauty, a large portion of the land was sold off and the proceeds used for this purpose. Currently the land, all the way back to A308, is now private homes and property.
You are looking for a Nano cache which only has a log book inside.