This is a physical cache which you can find after completing the Geocaching Adventure "The Lower Stratton Loop". There is not a physical cache at the posted coordinates. The Adventure takes in 5 iconic Stratton locations. You can view the Adventure Lab here.
If you already have the App then search on nearby Adventures to find "The Lower Stratton Loop". The whole Adventure and this bonus cache should take you no more than 40 minutes, if walking, to complete.
The coordinates for this physical cache will be revealed after you have completed all five stages of the Adventure Lab. You will need to be stealthy at GZ as the area is quite busy during the day. Two nearby Adventure Labs well worth investigation are "God's Wonderful Railway - the Swindon Story" and "Roundabout Swindon".