A little addition I found via twitter. From The Dialect Blog.
Edited to add: This is, of course, an addition to the English Accents in My Head post.
Edited to add: 3/19/13:
As this is a popular post, I would add that the Crawley family speak, as Dialect Blog says, in Received Pronunciation. The servants tend to speak in Yorkshire accents (I know that Joanne Froggatt, for example, who is from Yorkshire, said that her Anna accent is a slower, period Yorkshire accent. Sophia McShera is also from Yorkshire, as is Jim Carter). Other servants speak in generic northern English accents or Lancashire/ Cheshire/ Manchester accents.