[Alan]

There seems to be a lot of emphasis on colouration in this pic, but please remember unless your monitor is calibrated, it's likely that each of us will be seeing slightly different colours. As an exorcise, try de-saturating (convert to greyscale) the colours in photoshop or similar, and see what impression it gives you then!

I still stand with my original thought (although Imaginos mentioned it first!), that being it's a Garden Warbler.