This weekend, the Royal Navy was offering public tours of HMS Defender, one of their new-generation Type 45 destroyers. It's an astonishing ship: about 8,000 tonnes of steel and high-tech equipment designed to defend an entire fleet against air and missile attack. There's another type of attack it's more vulnerable against, though: the sea mine. And by luck, there was a good example of mine defence docked a little way upriver...