Siberian Chiffchaff -Back Again

An early start this morning secured three noteworthy birds, the first was a drake Pochard, this is the first I have seen at the fen this year. The second I did not clinch but sounded like an interesting bird I would like to see. The third was given away by the classic peep call; a Siberian Chiffchaff, I was tipped off about its presence yesterday (thanks!). It is the same individual that I found on 23rd December and funilly enough is in almost exactly the same spot too, so where has it been for a month?!
The bird is far from a classic Sibe and shows far more of a yellow hue than I would expect, however current thinking is that the call is perhaps the one most important feature and this one certainly had the right call (thank god!). It was in company of a common chiffchaff too so it was good to compare the sounds. Unfortunately they were both calling from the same area when I was taking pictures so I'm pretty sure this is the right bird! The bird with the yellow vent/flanks is the bird that was peeping! As Justin also commented on- the front half looks good, the rear not so good! If it were not calling I'm not sure I would give it the time it deserves! There was no sign of it a few hours later, so has it gone for another month?