A chiffchaff.
I may have seen these before but have always struggled to identify warblers - by the time I got back to a bird book I'd forgotten the song or couldn't translate the description into what i'd heard. Armed with the RSPB web site, the BirdNet bird call recognition app, and a digital camera with 300mm lens meant, I got a confident observation of that small brown bird cheeping away at the top of a 50 foot tree.