Knock John, like Shivering Sands, was a Maunsell Sea Fort in the Thames Estuary. It still stands today. All its ladders have been prised off to ensure it can’t be used as a smuggling stage… although it’s worth noting that the big guns weren’t taken off it until 1992. In 1965, it was taken over and used as a pirate radio station.
(We like our pirates around here. The creeks of shoreland Essex were the byways for pirates all the way into the 19th century, after all)
Radio Essex broadcast for a little over a year. They may have been the first British radio station to broadcast the likes of John Lee Hooker, I’m not sure — I know they played a lot of blues and R&B that wasn’t getting much attention elsewhere. I’ve read that Radio Essex was in fact criticised for being "weird" in 1966. Roy Bates, who set Radio Essex up, later decamped to the sea fort Roughs Tower, which you may know better as the principality of Sealand.
SHIVERING SANDS was my first POD book. A year from now or thereabouts, KNOCK JOHN will be the second.
(Superb image of Knock John by Richard Brown, found on a Flickr search)