Interesting CHIFFCHAFF at KLAS Kings Langley Fishery near Grand Union Canal

From the path between KLAS Kings Langley Fishery & the Grand Union Canal this morning I was alerted by a whistling call that sounded alot like it was some sort of waterfowl, very nearby in the margins of the large lake. It sounded alot like a young juvenile Coot, which would be pretty unusual at this time of year. On searching the edge of the water there was no sign of where the call was coming from but I could clearly hear it so I was baffled, but then right infront of me in the branches overhanging the lake I noticed a CHIFFCHAFF associating with a GOLDCREST & a LONG-TAILED TIT flock, but that call wasn't a regular Chiffchaff! I moved to view it better through the branches & it became aware of me, & with that the call stopped, though it continued to feed close by. It then struck me was how light the bird was, almost white below the belly to the under-tail coverts, whiteish throat, quite pale cheeks & generally washed out aroung its head, more brownish-grey than normal. But then how its flanks & some of its breast looked an obvious warm buff tone, in fact if I were to describe it to the extreme, compared to the bird I had seen here two days ago, which I would describe as 'dark green', this bird looked 'pale orange'! It was hard to judge the finer tones as it kept flitting behind branches & going against sky, then bark, then water, but the greenest part as far as I could see was its flight feathers. It had quite an obvious beigeish supercillium (eye stripe), more obvious above & behind the eye than I'd expect, & it somehow looked longer than a regular Chiffchaff, or at least its head looked longer as it appeared to have a slightly longer bill than normal & a less 'squashed-in' face.

With the general appearance of the bird & the mystery call which seemed to come from it, is it a SIBERIAN CHIFFCHAFF?

Before it dissapeared with the flock I attemped to grab some record shots to look at later, but it was extremely difficult through the branches & against the changing background. Below are the rubbish pics I did manage but the lightness & colour doesn't show so well in the more detailed shots as they are against the sky, the really really terrible shots on the other hand do atleast give an idea of the tones as they are against a more neutral, less bright background. An interesting detail that they do show is that the cheeks are pretty light for a Chiffchaff with a thin dark border.

Note the buffish supercillium, pale cheeks with thin darker outline, lightness of underneath & throat, greyish back of head.

Buffish supercillium, whitish throat, warm tone of flanks.

Olive tone in primaries, warm tone of flanks, greyish-brown above, whitish below.

Whitish throat & upper chest, pale cheeks.

Grey-brown crown, pale cheeks, whiteish throat, buff sides to breast.

Colours not accurate due to lighting but still shows buff tones to flanks & breast, whitish underneath,
grey-brown mantle, olive tones in primaries.

This pic is dark & grey as it was against bright sky but it does show pale cheeks with thin dark border.

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