Hi Matt,
My first thought is a solitary sandpiper (Tringa solitaria). These can look quite black & white from a distance, have light underwings and a white rump patch. Here in Holland they can be seen most of the year, usually associated with waterbanks. I have no idea how likely it is to encounter these birds in Britain though.
Cheers,
Tom
PS A painted snipe (it's nearest breeding grounds are in Egypt) would be a mega-rarity in Europe!