In 1647 when this lock was newly constructed the water level was much lower and those navigating the river had to pay the nearby mill to open their sluice and provide extra water for them to approach the lock.
However, boaters quickly complained the miller would keep a look out for boats and deliberately shut down the mill to stop the flow of water and extract extra money. Those who didn't pay were left on the dried up river bed until they stumped up.
The story is very likely true - mills at Abingdon and Sutton were reported to do the same, and by 1652 legislation was introduced to force mills to change a flat fee for passage,
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The good news is you won't need to worry about unscrupulous millers any more, but you may have to employ chicanery of your own to avoid muggle attention during summer. This micro cache only contains a log, please bring your own pen.