Flying wader ID

Hi all, 

About a week ago, I arrived at Amwell in Hertfordshire and as I started scanning I saw a medium sized wader in flight over the water. Unfortunately it didn't want to land and kept flying, eventually high and away to the south-east. I'm not an expert at wader ID at the best of times so I'm unsure what it could be, but what I noted was a white patch on the tail, some white markings on the wings, an overall medium brown colour and a darkish medium length bill.

Having flicked through the guides and some photos on the internet, the only one that really seems to fit the description is Curlew Sandpiper. It seemed too pale and marked for Green Sandpiper, the bill didn't seem long enough for a godwit, and dunlins (of which there's been a couple about) seem to lack the white tail. Are there any other candidates, or am I able to put it down as Curlew Sandpiper?

Many thanks,

Mark