The Naval, Military and Air Situation up to 12 noon on 21 March 1940 as reported to the War Cabinet:
Naval Situation General review
The principal event of the week has been the enemy air attack on Scapa. There was no U-boat activity until 20 March, when neutral casualties were reported off the North East coast. Attacks on East Coast shipping continue.
2. At dusk on 16 March an attack was made on Scapa by 15 enemy bombers operating in small groups, and continued from 7:45pm until 9pm
3. It is reported that only the first wave of five aircraft attacked ships in the harbour, dropping about 20 bombs. This attack came in low and climbed on reaching Scapa Flow to make dive bombing attacks on the fleet. H.M.S. Norfolk was hit in the quarter deck and holed by a near miss aft, 4 officers being killed, and 4 officers and 3 ratings wounded. The damage to the ship necessitates docking, but she was capable of steaming at 10 knots, and has since arrived at the Clyde. H.M.S. Iron Duke (depot ship) was also damaged by two near misses, and one other capital ship was attacked but not hit.
4. The remaining aircraft dropped a large number of high explosive and incendiary bombs near Hatston Aerodrome and Bridge of Wraith on the road between Kirkwall and Stromness. About 120 high explosives and over 500 incendiary bombs were dropped in all. Eight craters were made in the airfield at Hatston, some 800 yards from the hangers, and one bomb fell near a bomb store. No other damage was done to air force objectives, but two cottages were damaged at Bridge of Wraith, one civilian being killed and seven injured.
Protection of Seaborne Trade
13. Since the commencement of hostilities 13,673 ships have been convoy. 982 ships from Gibraltar, 963 from Halifax, 498 from Sierra Leone, over 1000 ships to or from Norway and 5739 up or down the east coast of England. 25 British, one allied and two neutral ships have been lost in convoy by enemy action, a loss equivalent to one ship in 488.
Military Situation : Western Front
24. Enemy patrols have been a little more active, particularly in areas west of the Vosges. Enemy artillery fire has been local and unimportant.
25. A survey of recent reports indicate that work on the northern extension of the Siegfried Line is to be resumed with renewed vigour directly the weather is suitable. Air photographs taken on 11 March reveal a forward line of defence is under construction in advance of the Siegfried Line, in the area south of Saarbrucken, these consist of a number of concrete forts behind wire and two rows of anti-tank obstacles.
Air Situation : Royal Air Force Operations: Attack on enemy base at Hornum (Sylt)
30. The main operation of the week, namely the attack on Sylt carried out during the night of the 19th-20th of March, was the largest air attack undertaken by either side since the beginning of the war. The night was one of clear moonlight, with areas of cloud over the North Sea, becoming somewhat denser towards morning. In all, 30 Whitleys and 20 Hampdens were dispatched, out of which 29 Whitleys and 17 Hampdens reached the target; of the remaining aircraft, three turned back having to technical trouble, and one was unable to locate the target. The first aircraft reached Sylt at 20:00 hours, and the target was from then on attacked at intervals during a period of six hours in all. One of the Whitleys is missing and was probably shot down by enemy anti-aircraft fire; two others were slightly damaged, but the rest of our aircraft all returned undamaged to their bases.
31. The object of the attack was the seaplane base of Hornum at the southern end of the island of Sylt.
Forty 500 lb. bombs, 84 250 lb. bombs and 1,260 incendiary bombs were dropped. The height of attacks varied between 10,000 and 1000 feet, the majority taking place place at an average height of about 4,000 feet. Reports of the damage done are not yet complete, but several hits are reported on hangers and close to a light railway, oil tanks and a sea plane jetty. Two hangars were set on fire and thus gave a clear indication of the objective to the remainder of our attacking aircraft.The only opposition met was with from searchlights and anti-aircraft fire. Only two enemy aircraft, a Messerschmitt 109 and a Heinkel floatplane, were encountered.
See TNA CAB 66/6/35