This looks to me like a knot in non-breeding plumage. There is a slight suggestion of red on the photo but that may be an artefact. The size (slightly larger than the red knot), structure (bulky body, small head, longish decurved bill) and plumage pattern (obvious breast band) make me think that Jacky's initial suggestion of Great Knot is correct.
Among other things, Grey-tailed Tattler should show a slimmer body, straight two-tone bill and usually barring on the underparts.
