I'd been getting worried - no sightings for a couple of weeks, and a confusion of prints that looked more ratty than anything. (The trouble is, in soft mud, rats' feet can splay out to make a print that's almost as star-shaped as a water vole's. It's very difficult sometimes to tell what's what.) Then tonight this rat, cheeky as you like. I've included a close-up of the tail to show how long and naked they are - water voles' are furry and shorter.
I was about to give up in disgust when I spotted a vole's nose peeping out from the vegetation near the bridge. The immediate giveaway, aside from the blunt nose, is the distinctive chocolate brown fur. Rats can be brownish as well as grey, but they're lighter in tone.