Reed Warbler
Acrocephalus scirpaceus

Reed Warblers are summer visitors to the UK and are probably more often heard than seen, hence the sketchyness of the photo. Below is a recording of the Reed Warbler's song (click the play button to start the track).
 

1: River Itchen, Southampton, 24/05/2006.
Recording: Dawlish Warren LNR, 19/06/2006.