Apparently the professional way to sex voles is to pop them in a Pringles tube and have a good look. Otherwise, it's down to whether there seems to be a bit of a ruff round the neck; if there is, it's a male. It's a subtle distinction, though, so unless a male and female of approximately the same size are standing helpfully together, it quite hard to tell, especially this time of year when some of last year's adults are looking pretty moth-eaten.
That said, I got a good look at the vole on the right and I reckon it's a male. The other one chased it away and it swam along and under the bridge all the way to the Tesco end culvert, where it had to negotiate another (new) discarded trolley. It ate some willow catkins, and it was while it was sitting hunched up that I spotted the ruff.
After that it swam back upstream against the current, and had a struggle, even though the water's low and slow just now. Water voles don't have webbed feet, and are easily swept off course. Several times it hopped up onto the bank and ran along the soil as the easier option.
I'd love to know how many breeding pairs there are along this stretch. You're supposed to be able to estimate by counting latrines, but that's if you can spot them!