The bird is tiny, despite the image being heavily cropped. Even though I know where it is, I find it hard to spot as its plumage merges with the varying dark tones of the shadows and lichens of the tree trunk (it’s very near the centre of the image).
It’s a treecreeper, creeping up one of the trees in West Park. The bird’s presence had first been disclosed by the song recognition function of the Merlin app on a smartphone.