(Northern) Chiffchaff
Phylloscopus collybita

Chiffchaffs used to be solely Summer visitors to the UK, but are staying over in Winter in increasing numbers, which are added to by continental birds deciding to stay here instead of flying to Africa. The majority still migrate from Africa each year. They look superficially similar to Willow Warblers and other Phylloscopus warblers, but are easily differentiated by their call, from which their name derives, which roughly translates as 'chiff-chaff-chiff', repeated several times.
 

1: Colebrook Lake South, Moor Green Lakes LNR, 29/03/2007.