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I thought it was nearly impossible to tell the two birds apart. The only certain method of identifying the bird would be to listen to it's song.
Not at all. As I suggested above, Willow would have a longer, clearer supercilium, the legs would be pale and the amount by which the primary feathers extend beyond the tertials would be greater. That latter point is the best feature.
Often, on iSpot, these features are obscured and then it is best to be cautious, but this photo shows all the features very well, so ID is easy.
My Flickr photos...
Lat/Lng: 52.8301, -0.4272
OS grid ref: TF060270