Thanks Paul.
There were two other points about this bird which, although not diagnostic, did separate it out.
Firstly, the shape was more slender than the dumpier shape of the Red-throated and Owen's comparison photos show this clearly.
Secondly, the bird was feeding in the mid to upper storey of trees, unlike Red-throated which normally skulks low down in the bushes.
Both these points are noted by Nial Moores on the Birds Korea website (www.birdskorea.org) under the ID section.
I have just come back from three days on Po Toi, unfortunately the bird was not seen, it appears to have left after the weekend.
Given that this is the second record on Po Toi this year, and there are records from Korea and Japan, I can't believe the eastern limit for Red-breasted Flycatcher is the Urals as most texts suggest.