Hams Hall, Warwickshire, UK, December 2016. Photos Alan Dean.

© A. R. Dean

Note prominent yellowish-olive fringes to remiges, rectrices, wing-coverts and scapulars : far more evident than in 'classic' tristis (see main Gallery images). Also yellow tinge in upper eye-ring.

© A. R. Dean

Comparison of  'Siberian Chiffchaff' type with  'fulvescens' traits and Common Chiffchaff of nominate form collybita.
Note generally tristis-like grey-brown upperparts and off-white underparts of 'fulvescens' bird, lacking the overall more-colourful, olive and yellow appearance of collybita.
However, 'fulvescens' bird also has  pronounced yellowish-olive fringes to remiges, rectrices, coverts and lower scapulars plus a yellow tinge above eye.
The extent and prominence of 'misplaced' olive and yellow implicate introgression from abietinus and would preclude this individual being logged as an unequivocal tristis.