The Adder in the photo above is a melanistic Adder sometimes called a black Adder. This particular snake was born with an increased amount of black pigmentation in the body (melanin), but if you look carefully you can still see the slightly darker zig-zag pattern along the spine. It was spotted in 2008 at Havant Thicket in Hampshire.
These darker forms occur more frequently in adders than they do in the smooth or grass snakes and could have advantages over the standard colour scheme. It's likely their colour offers better concealment (a cunning plan when hunting), and also their bodies warm up faster when basking.