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Not an easy one to be sure of from the photo, but it looks like a Chiffchaff - and if it was singing like one you've got my agreement!
chiffchaff and willow warbler are so similar, its difficult to be sure
I would hesitate to make an identification from this picture, the problem I have is with the legs. If they are dark (black) then it is a chiffchaff however if they are brown then it's a willow warbler, the legs could be brown but in shadow. Where the sun is catching its right leg it looks very light to me.
I agree that it is not really possible to be certain of the ID from the features visible in the photo. The photo does show that it is a 'Chiffchaff like' Phylloscopus warbler though, and as the song is very easy to recognise - and diagnostic - the observers statement "It was singing so I know it was a Chiffchaff" should be taken into account IMO.
Leg colour is not diagnostic BTW, Chiffchaffs do sometimes have pale legs.
Chiff Chaff are unmistakable by song. If the observer has heard it singing then it is a positive ID. Photos can be confusing for all sorts of reasons and I've had difficulties identifying some birds on this site that in the field would be easy.
Just go out there and do it!!!
Lat/Lng: 53.2, -0.9
OS grid ref: SK7471