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I suspect that Chiffchaff is correct.
These are notoriously difficult without a view of the wings, showing the length that the primaries project beyond the tertials (far greater in Willow than in Chiffchaff). Size variation between the two species is great and there can be overlap between female Willows and male Chiffchaffs. Leg colour is usually distinctive, but there can be overlap there, too, as too in the upperpart colouration. Bird ringers differentiate between the two easily, on the basis of the emargination of the primary feathers...but you cannot see that in a photo!
There will be plenty more tricky ones as the autumn approaches and the young birds get everywhere.
My Flickr photos...
Lat/Lng: 51.7, -0.8
OS grid ref: SP8310