Little Brown Job

This LBJ is in fact a chiffchaff (Phylloscopus collybita).
It closely resembles other warblers, particularly the willow warbler, but fortunately it has a very distinctive song call* and was vocally active at this time (see 'extra') so I can reliably identify it as a chiffchaff.

*  In my opinion, the chiffchaff's call is not tuneful enough to merit being called a 'song'.