There was a full-length shot, too, but I didn't post it as I know there are a few children who look at my blog and I didn't want to distress them. The body was intact but a bit manky.
If you ever see another unidentified dead mammal like the one you posted, try and get a look at its teeth.
I'm also fascinated by the water vole. I volunteer for SNH and we carried out a water vole survey recently, it was one of the best and most interesting things I've done for a while. Fascinating creatures!