Listening to the recordings linked by Annika, I still think the warbler was Japanese Leaf, xanthodryas.
To me, the song was a series of about 7-8 notes with a 'pumping' quality. This is similar to some of the phrases on the website link (especially in the first xanthodryas recording). This song reminds me a bit of Dusky Warbler in pace (but with slightly different structure of notes). The recordings of borealis comprise a longer series of more musical notes, the pace and quality reminding me of Lesser Whitethroat (I have also thought this when I have heard them on breeding grounds)
I acknowledge that most phrase in the recordings of xanthodryas include some slightly different notes, but this does not always seem to be the case. The song of the bird on Sunday was very similar to the birds recorded in Hong Kong last year, which were considered to be xanthodryas.
The call also fits closer to the call of xanthodryas, seeming to be more of a 'squelch' containing distinct strophes, slightly slower and lower in pitch than the call of borealis.
I hope this makes sense. I find it difficult to describe the differences in call/song. The bird was very close and calling/singing frequently when I saw it - perhaps I should have tried to get a recording then, as it seemed to be further away and more retiring when I returned a bit later.
