Greenshank or Marsh Sandpiper?

I saw and photographed this bird on April 2nd at Lymington in Hampshire. It's clearly a wader of the genus Tringa, which includes such species as the Redshank, Greenshank and various "sandpipers" (a generic term for any smallish, slim-built wader which encompasses species in many genera), but which one?



I posted the above pictures on BirdForum, saying:

Greenshank seems the nearest thing to this in my field guide, but i'm a bit uncertain because of the following:

a) the legs look grey rather than green

b) the field guide [The Hamlyn Guide to Birds of Britain and Europe, 1989 edition] describes Greenshank as "our largest and most robust Tringa". This bird was chased off by a Redshank, running right behind it, and looked just about exactly the same size, but if anything slimmer and more "fragile" looking (also slightly longer-necked and -legged, but my guess would be it weighed less than the Redshank).

When it flew away it had a very noticeable white rump and tail (looked all plain white, no visible barring, to me).

The other possible species suggested by people on BirdForum was Marsh Sandpiper, which is a fairly rare species in Britain, unlike the relatively commonplace Greenshank. However, the Redshank-sized bird i saw seems to fall awkwardly between Greenshank and Marsh Sandpiper in size (the Hamlyn Guide gives total length figures of 31cm for Greenshank, 27cm for Redshank and 23cm for Marsh Sandpiper).

There were lots of Redshanks about, which seemed fairly bold and aggressive, in contrast to this bird which seemed wary, timid and solitary.

I wasn't able to post my one photo with the "mystery Tringa sp." in the same frame as the Redshank on BirdForum because of their file size limits on attachments, but a BirdForum member called Pete Wragg sent me a message saying:

Hello,
I'm still not totally convinced this bird is not a Marsh Sandpiper.
Could you please e-mail me the picture with this bird and the Redshank in it.

Apologies to Pete for taking so long to respond, but here is the picture:


As you can see, the birds are at very different distances from the camera so a size comparison isn't particularly easy - however, i'll reiterate that when the Redshank chased the mystery bird off, they were beak to tail and looked very nearly exactly the same size, with the unidentified bird being perhaps a little taller and longer-necked than the Redshank, but making up for that with its slightly slenderer build.

After the responses my thought is that this bird was probably just a Greenshank at the small end of the species' size range (i'm assuming that birds, like people, do have "small", "medium" and "large" individuals within species, as i have observed in larger birds such as gulls and ducks), but it still seems like some doubt remains... any thoughts?

(edit: interestingly, not long after mine, there was another "Greenshank or Marsh Sandpiper?" ID debate on BirdForum here...)