A great find and wonderful image, Rosie. I think you have got the ID spot on here. They can be confused with a Wood sandpiper, but I am sure you are right and this is a Green Sandpiper.
If by chance you remember, or have any other shots of this with its wings up, and the underwing is dark, then that is 100% comfirmation.
If by chance you remember, or have any other shots of this with its wings up, and the underwing is dark, then that is 100% comfirmation.