As a fellow vole sufferer, I pursue an unrelenting guerilla war, using any/all legal means. They are enchanting little characters ( they will sit and watch you!) but they cause havoc. My plot is surrounded by 30 acres of rough grassland pasture so there is no chance of eradication, but some relief achieved by thinning resident populations.
Little Nippers, wired to pegs under cover , baited with peanut or peanut sized bits of apple, work. You must check at least daily (both for 'welfare' and anti-cannibal reasons- you may find skeleton within your trap.) To enlist natural assistance, corpses are left on 6' fenceposts which supply kestrels and a red kite. They are ,apparently , Europes most numerous mammal, so we must all have at least one.