A female bluebottle fly, probably Calliphora vomitoria (Diptera: Calliphoridae), based on the yellow beard, which appears to be characteristic of this species. The similar Calliphora vicina has yellowish 'cheeks', a grey beard, and an orange spiracle on each side of the thorax just behind the eyes. See this thread at Wild About Britain for more hints on identifying bluebottle flies.
This is the same fly from a different angle:
Photos taken between the castle and the river, Barnard Castle, County Durham, UK, on 2010-08-06.