Sallow Willow (Salix caprea). It is also know as Goat Willow. The Sallow tree normally forms a large bush but it can produce a respectable trunk and crown up to 10m, particularly when growing in woodland. Its twigs are quite smooth. Sallow has more rounded leaves than any other tree-forming willow, less than twice as long as broad; they are dark green and wrinkled above, grey and silky beneath. This is a female tree with as it bears long silky-green catkins which explode in May liberating white cottony seeds. This tree is common throughout northern Europe (except in the south west), in damp and dry woods and scrub.
Link: Google Images
Link: Google Streetview