Dunnock
Prunella modularis

Dunnocks are also sometimes known as Hedge Sparrows or Hedge Accentors. The latter is correct, but the former is wrong as they are not sparrows, but accentors. They are the only native accentor to Britain (there are thirteen species in total). The plumage is similar to a House Sparrow, except for the lead grey around the head and neck. They have a tinkling song, often from an exposed perch which can make them welcome garden visitors. Dunnocks in Britain, much like the Robin, are much tamer than those found on parts of the continent where they are often Alpine birds. They are roughly the same size as a Robin at 13-14 cm in length. While they look drab, it's worth checking out the Wikipedia article about their mating practices...
 

1: Moor Green Lakes LNR, Berkshire, 29/04/2013.
2: Chard Reservoir, Somerset, 03/03/2007.
3: RSPB Bowling Green Marsh, Topsham, Devon 01/12/2006.