An interesting bird and some very useful photos.
Everything about the plumage says Tibetan SP - long, white supercilium, large white forehead patch badly defined on the upper edge, pale lores, ear coverts similar in colour to crown, rounded breast patches, completely white flanks. The legs are very dark, the head is rounded and bird doesn't look particularly bulky.
But the bill looks really big for Tibetan, similar in length to Greater (as suggested by Butty). Despite this, the bill is fairly blunt-tipped and doesn't have the swollen nail that you often see on Greater (although columbinus often seems not to show this).
Overall, taking all features into account, I would say this is a Tibetan with a large bill.
Interesting that there are so many sand plover threads on the forum at the moment!