It's a Mahonia but I can't say which. There's Mahonia japonica, M. japonica Bealei, and a few cultivars which are crosses between M.japonica and M.lomariifolia. It could be any of these, there are other species but this doesn't look like one of them.
http://www.keith-allen.co.uk/g...
You may have heard of Oregon Grape, Mahonia aquifolium but the leaves are quite different. This may have set from a seed dropped by a bird!
