Bewick
Connoisseur of WTL
No offence but might you be slightly more specific with the leg and face differences. I’ve looked at images and still don’t see what makes it more likely to be a chiffchaff. Not quite sure what makes it a fledgling either.It's a scruffy juvenile Chiffchaff (this plumage is quickly moulted out) imo.
Western Bonelli's is actually one of the species I most frequently see in 'summer' here, and this one isn't one.
I'll leave that for you to find out.