I have not found a bomb store.
This building is identified in British Airfield Buildings (Vol 2) Page 34 as a Magazine to AM Drawing 4279/37. It may well have been used to store Detonator Bursters for small practice bombs.
Manby opend in 1938 and was an 'expansion' station and should have had a bomb store to AM Drawing 3054/36 but there are no signs of a store area on satellite imagery or old maps.
This location is unusually close to a hangar for an explosive store. Less than 150 yards!