Note: The account of the action on the night of the 22nd-23rd January, which is mentioned later in the article, comes from multiple sources. Many of these sources are confusing and contradictory. So the account is my best guess of the sequence of events.

In 1939 the Luftwaffe made a critical error, they dropped a new secret weapon, a magnetic mine, onto a sand bank off Shoeburyness in the Thames Estuary. This was quickly taken by the British and studied.
The actual mine, just before removal.
Now alert to the threat of these magnetic mines the British commandeered wooden hulled paddle steamers from the Thames, armed them with a few guns and mine sweeping gear and began to use them to patrol and clear the estuary. Part of their job was also to drive off German aircraft, and prevent the mines being laid in the first place. It was obvious from the outset that a better permanent solution was needed.
The solution was a series of forts made from reinforced concrete and sunk onto sand bars. These would give a permanent stable base for AA Guns. The forts were designed by the Engineer Guy Maunsell, who was an expert in concrete. He designed two types of forts, the army and navy forts. The former was a collection of towers laid out in a pattern identical to a land based AA battery, these forts held several AA guns, radar and search lights which effectively extended the flak umbrella across the Thames Estuary. Two of three three forts survive today, although Shivering Sands Fort lost one of its towers. Red Sands Fort has been taken over by Project Red Sands.
The Naval forts were twin towered affairs with a deck that held a pair of 3.7" AA guns and some smaller close in AA guns such as 40mm Bofors guns and machine guns. Of the three of the Naval forts, only one survives today, as the Principality of Sealand.
All these forts were constructed the same way, on a concrete pontoon which was then floated out to position then flooded. The pontoon would become the foot of the fort and sink onto the sandbar providing a stable base.
A Naval fort being sunk in position. Note the Crew on deck during this operation.

During the Second World War these forts claimed twenty two German aircraft and thirty flying bombs. They also claimed to have sunk an E-boat.
At 1753, on the 22nd of January 1945 several E-Boat flotillas were alerted to the likelihood of British convoys to Antwerp passing by and so were ordered to intercept. The boats of the 8th Schnellboatflotilla powered out of their bunker Ijmuiden, and turned towards the open sea. They encountered two separate aircraft and engaged them. The first after they took it under AA fire identified itself as a JU88, the second was a Whitley bomber, which they had a brief fight with to no effect on either side. They then reached their patrol zone.

Unknown to the Germans a British flotilla consisting of a control frigate (HMS Seymour) and several Motor Gun and Motor Torpedo Boats was closing on them. These patrol groups were set up to fight the E-Boat menace, the frigate, with its radar, provided command and control, as well as a massive whack of firepower. The motor boats were there to chase down the E-Boats.
At 0030 on the 23rd the E-boat flotilla picked up radar from a British ship, the sloop HMS Guillemot, and turned to attack. The E-Boats launched a single torpedo which missed and a brief, but intense, firefight erupted between the two sides. Hails of fire caused spurts of flame from the HMS Guillemot's bridge, and the return fire set a rubber life raft on one of the E-Boats on fire, unable to extinguish the fire they tossed the raft overboard.
Just before 0300 the E-Boats spot what turned out to be the British patrol group lead by HMS Seymour. The E-boats fired another single torpedo towards the British, who returned fire immediately, the E-Boats then broke away to the north before turning in for another salvo. This time the E-Boats put all eight of their remaining torpedoes in the water and again break away. At this point the Tongue Sands Fort begins to fire at the E-Boats with its twin 3.7" AA guns at a range of four miles.
On the lead E-Boat the Germans spotted three British MGB's approaching at full power and only 200m away, two more groups of MGB's are closing from North West. The lead boat turned due eastwards to avoid the closest British MGB's, and the rest of the line followed. However during the ensuring short but vicious firefight with every weapon on both sides blasting away at point blank range the E-Boat S-199 and S701 collided (many sources say this collision was between MGB 495 and S701).

The impact was so severe the crew of S701 was thrown to the deck, staggering to their feet the crew saw the boat they hit as a shadow in the water to starboard. The impact caused severe flooding but S701 was able to maintain her speed. It also left a magazine for the forward gun, marked S199 on the deck of S701.

Bursting clear of the fire fight the remaining boats headed for home. Although two incidents further marred the deployment as two boats ran aground at speed on the treacherous sandbars that infest the area, it was only the rising tide that allowed them to float off. A third E-Boat hit floating debris and sprung a leak. This effectively put the entire flotilla out of action. The crew of S199 was rescued by the trawler Neave. On the British side MTB 495 took one shell to the engine room and Able Seaman George Calder, aged 21 from Edinburgh was killed, and the only causality of the fight on both sides.

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