Some more research. There are two common kinds of vole in Britain (plus a few rarer ones). After a bit more research, I read that the one that might do tree damage, by chewing bark of young trees, is the field vole, Microtus agrestis. What they mostly eat is grass. They are more likely to damage the bark of young trees if the tree trunk has long grass around it. The other common vole is the bank vole, M glareolus. Bank voles mostly eat fruit, nuts, leaves, seeds and a few small animals.