Alderbury VRP is easy to spot if you know what to look for.
Approaching from Chilbolton you'll be flying along a shallow valley. On your left will be an ammunition depot. It has a narrow-gauge railway loop running round it with sidings disappearing into the base of the ridge on your left. To your right is another ridge. Salisbury Cathedral will be readily visible in your 2 o'clock.
Crossing the end of the valley in front of you is the main Southampton-Salisbury road and there is a very distinctive road bridge crossing it. That bridge is at the southern end of Alderbury village.