The fact that some people are suggesting Cetti's shows that people can be wrong.
As I said in my previous post, I have very recent experience of spring Sykes's, EOW and Upcher's.
Sykes's and EOW look incredibly similar to each other; the first Sykes's I saw was identified retrospectively on call, and had been down as EOW for three days.
This bird clearly has the correct wing structure for Upcher's, and in all photo's of the uppertail it looks dark with a narrow whitish outer tail feather. It has an enormous bill and is clearly a big bird. Olivaceous tends to have grey/bluish legs, Sykes's tends to have pinker legs (one bird we saw had quite bright pink legs); This bird does seem to have pinkish legs, which is at odds with the dark legs of Upcher's - to me the only anomalous feature.
Tail movements are pretty unique; the on Upcher's I saw I didn't see it, but Upcher's wings its tail. Sykes's dips its tail both above and below the horizontal, and also seems to flick it to the sides. Olivaceous dips it repeatedly down below the horizontal. Botted seems o have a nervous sideways jerk of its tail (from personal observations).
In summary, I'd say Upchers. If it's proven not to be, then Sykes's is the second probability.