Google Lens comes up with Asian Stubtail but I have no personal knowledge whether this is correct or not. However, the photos on eBird look pretty similar and it is a bird that occurs in Japan.
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Tony
My Flickr Photostream
?Arctic warbler.
Thanks for these TeeJay and Cris. I had come across both of these in my searching but plumped for leaf warbler because of the breast streaking. If I'd got a shot of the bird from the side the stubtail option would be easier to accept.....but I didn't!! I'm still uncertain nonetheless!!