(Cetti's Warbler) Any ideas what this could be?

(Cetti's Warbler) Any ideas what this could be?

Observation - (Cetti's Warbler) Any ideas what this could be? - UK and Ireland. Description: Just off the path in a more wooded area of WWT Barnes. I only took 1 photo before it flew away. Th

Just off the path in a more wooded area of WWT Barnes. I only took 1 photo before it flew away.
This is quite an old image, I was looking through my photos and realised that I never found out what this bird was.