Seen on pile of branches, with another on a separate pile. By the side of river, near reeds.
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Think I'm correct in saying that the pale 'eyebrow' (supercilium) is a particularly distinctive feature of the sedge, and as you mention the call/song the Reed Warbler's is much more rhythmic than these fellas chaotic rattle!
Feel free to look at my ongoing (hopefully improving) collection of pictures on Flickr:
Lat/Lng: 57.443, -2.2979
OS grid ref: NJ822392