Water Vole Arvicola amphibius 

Water voles occur mainly along well-vegetated banks of slow-flowing rivers, ditches, dykes and lakes. They are rat-sized with blunt nose; dark chestnut-brown to black fur; short rounded ears; hair-covered tail, which is about half length of head and body. Image: Margaret Holland (Flickr).

Find out more: The Mammal Society, Suffolk Wildlife Trust, The People's Trust for Endangered Species,  iNaturalist