I think this is a Andrena cineraria - ashy mining-bee
A very distinctively marked ground-nesting bee that often nests in large aggregations in lawns and short turf. Common and widespread across most of Britain and Ireland, and apparently increasing in both range and abundance. The bee flies from late March to mid-June
Info from http://www.bwars.com/index.php?q=content/beginners-bees-wasps-andrena-cineraria
