There's an article on the BBC site about Water Voles.
Apparently Water Voles have a home range of a few hundred square metres but a study at the University of Aberdeen has shown that Water Voles can travel surprisingly large distances.
According to the study young Voles wandered far from their home range.
Professor Lambin said:
Those animals typically have a home range of a few hundred
square meters, and we found them moving two to three kilometres, a few even
moving 15 kilometres between [the site of] their birth and their first
Apparently these movements allow Voles to exist in small density populations in Scotland.