Seems to be sunbathing. I've seen a great tit doing this too.
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My Flickr photos...
I've seen two more instances of this today - I haven't looked at the pictures yet, but I think one was a young robin, and the other a willow warbler/chiffchaff (I don't think I'll ever get the hang of telling the difference! I've said it before and I'll say it again - I HATE warblers... ;-O )
I'll put the pics up in the next day or so.
Lat/Lng: 52.6405, -0.68
OS grid ref: SK894055