Chiffchaff or Wood Warbler?

I'd say Willow Warbler. The call would also give it away but the lighter brown legs are a good indicator. Chiffchaff has dark brown legs. Good spot as depending on where in the country you are, Chiffchaff are usually far more common than Willow Warblers these days.

If it helps at all, these two are definitely Willow Warbler (they were singing when I saw them).  I say two, but I suspect they're the same bird, as it's the same small tree in both photos.  There was a Willow Warbler singing there most days last summer, and I never heard more than one in that area. 

Hi

I'm pro Chiff on this one-
the eyering is distinct from the cheek ( although there is some shadow)
the primary projection is short - compared to the secondary length - even allowing for fore-shortening
I think the legs are dark enough for Chiff allowing for the contrasty light
The overall shape is Chiff not Willow to my eyes

If you put Common Chiffchaff into Wikipedia there's a nice photo (with a song link )that looks virtually identical

S