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Not an easy one, for me. Leg colour I find hard to judge (and can be of limited use on some individuals), and the key feature of primary projection is not visible. The eye stripe and supercilium look quite strong and the cheek looks palish, both features of Willow. Plenty of Willows are already back for the summer, so the assumption can no longer be that it should be a chiffchaff.
Tricky one, I think.
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I would be inclined towards Willow too, the leg colour is relatively light (I know this is not diagnostic) and I agree with ophrys on eye stripe etc.
Also can confirm Willows about, one was singing away at the London Wetland Centre last week. Spent some time chasing it around but it was very good at avoiding the camera.
Lat/Lng: 51.37861, -2.42675
OS grid ref: ST703644