Hello Rees,
presuming it is a recent picture (march 2024?), than its often good to start with a Chiffchaff and only then to think of a Willow Warbler. Or that you hear a song or a call? Or did you identify the bird as a Willow Warbler in the field? You know, I trust original observers more than a single picture.
Anyway, all dark and fine bill is better for a Chiffchaff, many WW have a slightly stronger one with a pale, sometimes even pale orangey base.
And surely biased by date, I find the dirty olive upperparts and ochre-yellowish hues on the breast better for a Chiffchaff.
And does your bird has a short pp? Yes, I think so, but its hard to be sure. Do you have more pictures?
As said, going only by the single picture and the date, I think its a Chiffchaff. But its hard to exclude a WW with 100% confidence, at least for me.
But its surely one of those two species imo