Hello Brian,
with a little bit of whisful thinking and after brightening the image, olive-greenis hues on the upperparts and some yellow bits on undertail-coverts, vent, supercilium and breast are visible.
Therefore its a
Phylloscopus warbler, with Chiffchaff and Willow Warbler the most common choice.
Although it looks slightly better for a Chiffchaff imo, I struggle to exclude a Willow Warbler with confidence.
Your bird seems to be singing, do you remember it? As Chiffchaffs have monoton songs, I am regular biased towards Willow Warbler in such cases (no offence you know).
And please note, that dark legs are rarely present in WW (while Chiffchaffs with truely
clear pale legs, not speaking of picture artefacts and backlits situations are extremly rare). And that true leg colour is difficult to judge here, at least on my actual device.
Do you have more pictures? If forced, I would be in the Chiffchaff camp, but with a bad feeling.
