Sorry GazB,
I use the term "pitfall picture" to describe, when a bird is difficult to id with one (or more) picture(s), because features visible are appearantly changed due to light conditions, angle of viewing, jpg-artefacts etc.
I will try to explain this with your Goldcrest (I hope this is helpfull, if not, my usual apologies are that I am not a native speaker):
Goldcrest is a tiny, round, restless bird. Your bird in contrast seems to be sleek, slender, "long-necked for a Goldcrest", with a head that is seperated from the body. I assume the reason for this is that your picture only represents a fraction of a second, as opposed to a normal observation represents an impression gained over a longer period (which often lasts for one or two seconds in Goldcrest).
Your Goldcrest seems to have warm buffish, orange-brown tones to the body, with supercilium and belly having a yellowish wash (those are the colours that appear on my screen). I cant remember having seen a Goldcrest with that colouration. More, wouldnt you like a (suspected) Siberian Chiffchaff to show such smooth, warm rusty-buff ear-coverts like your Goldcrest?
In summary, the colours of your bird seemed so unusual to me, including total lack of grey tones to the head, that I assume this is all caused by light conditions and/or camera-settings.
I have done a quick google-search and found a picture with warmer brown ear-coverts:
https://www.littlepeckers.co.uk/s/Goldcrest, so time for me to search for my first "Siberian Chiffchaff-ear-coverts Goldcrest"!
Have you ever seen a Goldcrest with fine, sharp streaks to the flanks like your bird? I cant remember seeing this feature in a Goldcrest before. But as the bird looked like a Goldcrest to me, I decided to skip this feature, but I didnt have done that, if I didnt know the presumed location (England).
Its hard to find a Goldcrest where the black borders to the crown doesnt meet on the forehead. This connection bar is sometimes weak (like rubbed away from a pencil painting with an erazer), but it is clearly there (adding to the typical Goldcrest face with a large dark eye enhanced by a pale ring around it and a typically, triangular shaped paler area in the loral area. On first sight, your bird seemed to lack a dark connection bar on the forehead and therefore the head gave an unusual jizz to me. But on a closer second look, it is there, but burned out in an overexposed picture.
Conclusion? I hope, you got a feeling what I wanted to describe with the term "pitfall"-picture. Imagine, there is a species of warbler somewhere in SE-Asia, that I dont know and not aware of. It is similar to an (European) Goldcrest, but has a slender, sleeker, more warbler-like build and has warmer buff and yellow colouration and sharp streaks to the flanks, and a slender spiky bill for an Phylloscopus-warbler ? Its the "Tinkerbell-Warbler" (sorry, couldnt resist that). This is the reason, I normally just write comments here, when I have own experience with a species. And more: thats why I am so gratefull for comments in this area, like for example Nutcracker, who mentioned the existence of an American ssp of Goosander to me. Thanks again for that, Nutcracker!