Good catch miken!A Firecrest.
Along with the Goldcrest it's the smallest UK bird. Never an easy subject as it is quite restless.
This image obtained after six two hour sessions.
1DX2 + 100-400mk2 +1.4 1/500. f8. ISO 10,000
I encountered this one in a very early morning during my vacation in Europe this summer (and it was one of my "for sure" targets). In deep shade, against bright background... Camera on tripod. The speed was about 1/10 or less after the +EV corrections (at ISO far beyond the acceptable abilities of APS-C sensor). I knew there is no chance to get the (+/-acceptable) photo - took it just as a prove that I have seen the bird. Otherwise, back in the time (Huh - 20-25 years ago?!) I have seen that bird (as well as the Goldcrest) in very nice light and surprisingly close to me (same mountain). I was not taking photos of birds at that time!