Most people, if they think of it at all, regard the Spree as an urban waterway, the river that runs through the centre of Berlin. But there’s much more to it than that. About 50 miles (80km) south of the capital, it splits into a network of rivers and shallow channels known as the Spreewald, a Unesco Biosphere Reserve that’s Germany’s answer to the Norfolk Broads. It’s a beautiful area, and ridiculously easy to access, either by adding a couple …