ID without photo

[crates]

Appreciate almost impossible to ID bird without pic. But yesterday had unidentified bird (tree suburban garden Essex). Unable to take pic as too flitty.
small completely yellow breast and back brown (almost sparrow/dunnock- like)
Does not seem to fit anything in my bird book!.Any thoughts?

[solus]

Juvenile Willow Warbler??

Can you give any more detail - what was the bill like, for example? Conical like a House Sparrow or thinner and pointed like a Dunnock?

[crates]

Thanks. That was it!
Should have said dunnock-like bill and was obviously eating insects from the tips of branches. Must be something good up there as same tree visited daily by flock of long-tailed tits past week. The guide I was using showed adult with white/buff breast. But since found image of immature which matches exactly.