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Assuming that it really is a white spot at the base of the bill, then I think you are right. Otherwise, the features can't be made out clearly enough to be certain.
My Flickr photos...
What else do you think it might be if not a white spot?
It's an unclear photo of an apparently young bird and it shows some features of Willow, like a diffuse bib and extensively white cheeks. I'm wary of interpreting features from poor photos. It probably is a white spot and therefore Marsh, but you'd also look at the photo and be fairly sure it has a large dark bib...but it can't have, if it is Marsh. In other words...a series of photos is really needed to be sure of what these features actually look like.
Lat/Lng: 51.1156, -0.6143
OS grid ref: SU970360