Another early morning at the Ministry of Public Relations, and another surprise.... I got onto a Yellow-browed Warbler and watched it for a couple of minutes, and then became aware of another phyllosc close by. This second bird looked somewhat bigger, with less "compact" proportions that the yellow-brow - it had a longer bill and tail. It displayed two well-defined yellow wingbars, and a strong supercilium, with the bill having a pink lower mandible with a dark tip. Given these features, I was reasonably confident that it was a Two-barred Greenish Warbler, though something seemed wrong - it appeared to have at least a hint of a central crown stripe (which 2BGW doesn't show) - was I screwing up an Eastern Crowned? Rather frustratingly it moved deeper and higher into cover and started preening, but was helpfully flushed by a Black-naped Monarch, and dropped down to give me excellent, pointblank views just above head height and I could now see that it did indeed have a quite well-marked central crown stripe, and two very obvious wing bars (so ruling out Eastern Crowned). At this stage I was very confused about what it was - I see lots of Yellow-brows, Eastern Crowned and Arctic Warblers in Bangkok, plus a few 2BGWs, but this thing wasn't any of those! I guessed it might be Blyth's Leaf Warbler, and indeed a check of the books once home, and consultation with Oriental Bird Images confirms the ID. However Blyth's has recently been split into three species including claudiae and goodsoni, and it seems pretty difficult to assign a bird in the field to one or other of these species, though it is thought that migrants in Thailand are claudiae (per Phil Round).
"Blyth's" Leaf Warbler is a non-breeding visitor to the Bangkok area, seemingly in small numbers, though I suspect it is under-recorded here.
EDIT: tried to get some photos of the Blyth's after work - whilst I did see it, it wasn't playing ball for the camera.