The Willow Warbler looks similar to the Chiffchaff, another "Leaf Warbler," but usually has pink/orange legs/feet rather than black, a slightly longer supercilium (eyebrow) and more skulking habits.
It also has a much more melodious song. Warblers on this page have been identified as Willow Warblers rather than Chiffchaffs largely on the basis of paler legs.
Positive ID as Willow Warblers by expert
Younger ones have yellower plumage
Unsure of ID of these in particular -- may be Garden Warbler ??