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For the reasons given by Mike, this is a Chiffchaff (although the primary projection cannot really be seen in the pictures given). The whole colour is rather dark for Willow Warbler and the face pattern is far too dark, with too short a supercilium and dark cheeks.
The second picture shows that the area around the base of the bill is clearly dark and the feathers appear rather matted. This is suggestive of a bird which may have come here from further south, after feeding on nectar, perhaps.
The following link refers to this...
Of course, they could presumably also pick this up from closer to home...?
My Flickr photos...
Lat/Lng: 53.42416, -2.50661
OS grid ref: SJ664920