Telling these two birds apart is very difficult in the field, but one of the main identifying features is the over all grey tones of the Siberian Chiffchaff. Today's bird showed quite well, if somewhat briefly, and was noticeably greyer than the accompanying Common Chiffchaffs. I managed to grab a very poor photograph, which I heavily cropped.
Siberian Chiffchaff (honestly!)
To see some much better photographs of other Siberian Chiffchaffs see the Birdguides website here.
As this bird is still a subspecies of Common Chiffchaff it can't be added to my Year List.