How do I know it's a water vole? Partly because I got a good view of it as it passed under the bridge, but also because this is typical w-v behaviour - hugging the bank, but keeping submerged. Rats prefer to swim with their heads out of the water, and fish wouldn't keep close to the side like this (not that we have any sizeable fish in this section of the Staggs Brook). So all right, no contender for Wildlife Video of the Year, but definite proof there's still vole action at White Lion Meadow.
PS To compensate for my inadequacy, here's a really good video from a colleague on the Wild About Britain forum: http://my.opera.com/Ukwildlife/blog/water-vole