Willow Warbler in song

The only way you can tell it from the Chiffchaff is by the song which is a loud repeating pattern going downwards (Turn on your Computer speakers) - it never makes the "chiff chaff chiff chaff" sound.

It is a Summer visitor which you will see (hear) from April to September.  Their population, especially in southern Britain, has undergone a moderate decline over the past 25 years making them an Amber List species.

Willow Warbler showing stripe above the eye