Although its a realy hard picture with much artefacts, at least for me, I am in the Reed Warbler camp for this.
Please note long pp, better leg colour for a Reed Warbler and dirty colours with an olive tinge.
I assume, that the original picture could change this in an easy identification, but this low resolution, "doubtfull colour-biased", artefact-filled picture, makes assesment of the relevant features difficult. I hope this is not regarded as an offense, but as an explanation, why I struggle to support my gut-feeling that its a Reed Warbler in this case.
Good point Roland, I must admit, I excluded Marsh warbler only by location. And by the fact, that I struggle to support my ID as a Reed Warbler against a Common Whitethroat with this picture (no offense, as said!)
I have no sound recording or video of this bird to be honest I just pointed the camera to where I thought the bird was and took a burst. I have attached an original of the burst. Don't know if it will help its under the half moon weed. Mike
I can't bring anything new to the ID debate (I thought it was a whitethroat too...)...however, although marsh warbler is undeniably rare, there have been a few breeding records up the east coast in the past few years (I recall one at least in Northumberland) so not totally out of the question that you'd find one holding territory at Walberswick.