Probably a Willow, but I am not certain, as the legs do look very dark. Chiffchaffs can be very yellow below, and the supercilium can be this long. The head looks rather rounded and Chiffy in shape?

The pale ear coverts look rather Willowy, though. They are not as obvious as they sometimes seem, to be honest.

I have withdrawn my agreement with Willow. Following ophrys comments I looked closer at the photo and what I had assumed to be very light legs (at a too brief first look) is a bit of dead bark/needle. I think the darkness of the legs is largely due to shadow but overall there is too much doubt to say for certain from this photo. Jizz wise it still seems Willowy to me though; shame we can't see the primary projection. Don't suppose you have any other pictures Patrick?

Thanks for the extra photo. To be honest, I just dont know any more. There is a huge amount of shadow thrown onto the legs in both pictures, and I think it would be hard to tell by legs alone. I bow to peoples greater knowledge !

The strong supercilium, pale centred cheeks and Pale(ish) legs all point to Willow Warbler, and although the yellow colour is typical for juvenile Willow Warblers, it is fairly unusual for young Chiffchaffs to be quite this yellow (and unmarked) below.