"... any Chiffchaff producing a typical tristis call should be a tristis, providing that plumage and bare-part colorations remain within the limits described..." van den Berg (2009).I have my doubts about our current approach to Chiffchaffs but, if this is where the goal posts are at, and apparently it is (for now at least), then I had a classic Siberian Chiffchaff in Avon/North Somerset/South Gloucestershire/some geographically schizophrenic location this morning. I was working at the time so, unfortunately, I didn't have DSLR or recording equipment with me, but I'll have another look next week. In the meantime, if you're in the Avonmouth area, take a wander down Lawrence Weston Road, the bird was showing very well (and heard to call) just past the incinerator site in the trees/bushes on either side of the road opposite the brewery distribution depot. In addition, two or three Ruddy Ducks were at the 'square pool' near Avonmouth Sewage Works along with a good dose of native wildfowl.
NB. As often seems to be the case nowadays the reference you need is from Dutch Birding not BB or Birding World, details as follows: van den Berg, A. (2009). Calls, identification and taxonomy of Siberian Chiffchaff: an analysis. Dutch Birding 31: 79-85.