Warbler

I'm guessing that this is probably a Reed Warbler, rather than a Marsh Warbler.  The legs appear to be paler than one might expect with a Reed Warbler and the birds underside has a slight tinge of yellow to it, but Marsh Warblers are such rare birds in the UK that I think it very unlikely.=.  

As you can see, it has managed to catch a flying insect of some sort and when I took this picture it was about to fly back into the reeds to feed it to its fledgling.