I don't think I've ever seen one of these before. If I'm right, my wildlife book suggests it's a Northern Water Vole. He was quite big (at least twice the size of a house mouse) and roundish, with a short tail. I took him out with a small skimmer net, and he scuttled off between some stones.
The swirling white stuff in the photo is Pine pollen; we're inundated with the stuff at the moment.