Chiffchaff ssp- overall lacking nominate collybita green yellow tones with olive grey upperparts, slightly more olive tones on secondary panel and tail, buff fringed dark alula and contrasting pale underparts- exaggerated in this image by the stronger light. The supercilium is atypically poorly developed in front of the eye. Bill and legs are black.
The greater covert bar is well shown in this image as is the slightly contrasting olive toned secondary panel. At this angle the bird appears colder toned (as above) with an overall olive grey appearance
Possible identifications of this bird are Scandanavian Chiffchaff (abietinus), Siberian Chiffchaff (tristis) or hybrid between the two (relatively unlikely as the hybrid zone is reported to be very small in comparison to the respective ranges) . Recent studies suggest that Scandanavian Chiffchaff are less likely to occur here (based on genetic studies from the Netherlands) than Siberian Chiffchaff. Plumage variation in Siberian Chiffchaff is reported to range from grey and white to brown and buff (which is complicated in field conditions due to changing perception with different light condiitons) with varying degrees of olive (usually around the secondaries, rump and tail).
Seems like that with a bird appearing in most lights more olive-grey and pale (like this one) rather than buff and brown it is probably necessary to hear it call or sing to have a better idea of what it is.
Hopefully we get to hear it call.