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Definitely a Phylloscopus: the bill is too short and fine, and the head too rounded to be an Acrocephalus. The legs look tan and the face pattern & paler base to bill make me think Willow Warbler is more likely than Chiffchaff, but there's nothing conclusive in there.
Flickr pics: http://www.flickr.com/photos/72243477@N00/
Looks like a juv at the right time for dispersal, so agree it is difficult without seeing more of the bird (primaries mainly). I've seen Willow Warbler families feeding well-grown young well away from their breeding habitat at this time so even that isn't a help.
Jamie from Briantspuddle
It's a moulting adult, isn't it...a juvenile would have much fresher plumage and would be yellower beneath.
My Flickr photos...
Yes, looks like an adult from the full-res image. Very scruffy, but not looking like juvenile-structure feathers - nor that dowdy look that the very young Phylloscs have straight out of the nest.
Thanks for all your helpful comments.
Ian, I had noticed your comment and have been thinking about it. I really meant the fledgling stage of juvs before they had the full yellow.
But I am trying to remember what the adults look like at the same stage. I can't think that I've ever seen an adult moulting - they go very quiet and hide, I thought.
Jerebarker makes interesting and good point about juv feather structure which I don't know much about!
Lat/Lng: 50.6537, -2.198
OS grid ref: SY860837
Observed by the side of a lake in dense vegetation.