I drafted a long narrative about this, but it's a bit much for the Gallery. So, briefly, I saw this bird ten minutes after the Willow Warbler posted earlier, and they seem quite different individuals. Among other things, this one is greyer and less yellow, and the primary projection seems longer. A generally smarter bird.
Dark legs usually signify Chiffchaff, but dark-legged Willow Warblers like this do occur.
Thank you for your kind comments on my pictures, and good birding!
John