Our field voles (Microtus agrestis) are having a winter break from their long breeding season. They don’t hibernate; instead they grow a thick, very dense coat to shield them against the cold, and their metabolic rate almost doubles in order to maintain their body temperature.
It used to be assumed that in cold weather field voles would retreat further underground the way moles do. However, the latest study has shown that this is not so: field vole tunnels are not deeper in the winter than they are during the summer and neither are they measurably warmer than the air temperature just above the level of the grass.
But down near its roots, grassland does provide a sheltered microclimate, where field voles are safe from wind chill and the worst of our weather. Here, they can go about their business undeterred by conditions that predators might find more difficult to cope with.
If the temperature falls dangerously low, field voles retreat to well insulated nests where they huddle together to share their warmth.
Southwick Country Park Nature Reserve 
I read somewhere there are more field voles than people in the country but I am not sure I have ever seen one. Where do they hide?
The Mammal Society says that there are 75 million field voles in the UK and Google says there are 67 million people – so you are absolutely correct. They live in tunnels just below the surface in open grassland.