With changes to what seems to pass for tristis, there are several candidates in the county. Here are links to the sites where claims are being made (it is apparently “scarce” rather than “rare”
This is the latest, possibly controversial, ID guide to “tristis”
A rough guide to plumage issues:
1 tristis varies from brown to grey looking above with/without olive bits and extra little bits of yellow can be present. Head plumage often distinctive.
2 Same bird can ‘morph’ appearing both brown and grey (to do with light ‘n stuff).
3 Brown/buff hues are limited to season. Many mid-winter and spring birds become greyer and can lose buffs and browns.
4 Choice for most autumn/winter Chiffchaffs is either collybita or tristis.
5 The term ‘grey and white Chiffchaff’ has been confusingly used. Originally applied to autumn birds completely lacking buff and brown tones (these normally being present to some degree on the head patterns of most tristis). The particular ‘grey and white Chiffchaff’ plumage intended by Dean and Svensson (2005) seems to be rare in Britain n.b. identifiable Siberian Chiffchaffs are much more common.
6 There are still occasional birds which defy easy identification due to plumage/sound peculiarities. These are worthy of the most careful study with the aim of collecting as much data on them as possible. They may be best left unidentified.
7 There is still stuff to be discovered particularly about the precise extent of any hybridisation between tristis/abietinus and the degree of introgression of abietinus characters into the western tristis population.