Talking of Shoeburyness. A friend of mine back in the 70's was first officer on a gravel dredger which was basically a large floating vacuum cleaner. The normal practice was to drop the suction pipe over the side and see what comes up (mostly for the quality, they know it should be sand and gravel). He found what came up was very good so he started dredging. Half an hour later he had a visit from the military security, he was in the restricted zone off of Shoeburyness. He was fortunate his boss only gave him a dressing down, he could have sucked up a bit more than sand or gravel!