This is a Common Sandpiper - the way the white of the flanks extends up towards the shoulder is often said to be the most obvious feature, but in the field there are a number of 'jizz' elements which make it clearly either this or the very similar North American sister species Spotted Sandpiper.
Having said that the smaller Temminck's Stint bears an uncanny superficial resemblance, in terms of the white area referred to above at least.
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