As always with these things, identification is some way from straightforward. General consensus seems to favour Acadian (E virescens) but, like the Blakeney Point Alder Flycatcher of 2010, different images seem to suggest different things. Compared to the Blakeney bird, the Dungeness Empid seems more green (rather than olive-grey) above, not too dissimilar to the upperpart colouration of e.g. Yellow-browed Warbler. Below it is somewhat yellower and, crucially, the bill is quite long and gives a downturned impression in some images thanks to a slightly hooked upper mandible (compared to straight/almost upturned in the Blakeney Alder). The dull light seems to inhibit these yellow tones showing so beware some photos where it looks greyer (and more Alder-like!) - just look at my shots below. At this point in time, Alder(/Willow) is considered to have been ruled out and the main emphasis of today's discussion, both across the internet and on site, referred to separating Yellow-bellied and Acadian, though a range of features seem to rule out the former.
A nice bird but one of those that, once the stress and adrenaline of the twitch subsides, you wish you'd spent a bit more time watching and had a bit more knowledge about at the time. That said, the weather was bloody grim this afternoon and I've had a nasty bout of flu since returning from Ireland on Sunday evening, so probably best not to have spent several hours in the wet.
Presumed Acadian Flycatcher, Dungeness, Kent, 22 September 2015