It would be useful if an expert or other experienced birders could comment on this one...
It's causing us problems!
I have seen every breeding species of Warbler in Britain and Sedge Warblers always strike me as a quite well marked bird. Some of the head markings shown on Sedge Warbler but not on this bird include: a long, broad supercillium which touches the bill, above the supercilium the head is manly dark a part from a dull brown crown-centre and a indistinct ear covert patch. The legs on a Sedge Warbler are a brownish pink colour, where as a Reed Warbler's are a plain brown.
Compare this bird to a Sedge Warbler on this observation: http://www.ispot.org.uk/node/279009?nav=users_observations
I would say this is a young Reed Warbler, though I could be wrong.
Thanks for your comments.
I just wondered whether the strong light might be masking features on the head more than is obvious. You may be right with Reed Warbler, though, but I do still think that it looks quite heavily marked on the wing/back for Reed.
We shall never know, but it's good that there are still birds to make us think!
My Flickr photos...