What's the scale? What are the measurements of the rail it's sitting on? Definitely not a chaffinch - wrong colour and has a stripe over its eye.
Some possibilities : yellow wagtail juvenile (prob not); yellow browed warbler; wood warbler; chiffchaff; willow warbler - my favourite option as has a darker stripe through its eye just below the yellow stripe; almost any of the LBJs. Size helps, and call, although most babies just say 'cheep'

. Where you live also makes a difference. Whatever it is, I've not noticed one round here
