It is not unknown for birds to have unusual plumage. I remember when I was a child many years ago there was always a few blackbirds with large areas of white feathers on them in Staghills Woods. I saw a blackbird in Bury this year that looked like a ring ousel as well, due its white chest band. It does make the commoner birds more interesting when it occurs. The BTO have a page for abnormal plumage which you should be able to find from the link below
Thanks for the replies gentlemen, the bird is similar to the robin in Derbyshire but has no white on its chin and more on the tail, I'll try and get a photo but I've not seen it for a couple of days, Cheers Tony.