I managed to track down a classic tristis Chiffchaff today at Helston Sewage works, prompting me to offer/recycle a few thoughts on identifying this race (comments and corrections welcome). First-off they're not that cold grey. If you find a really cold steely grey one, it's probably an abietinus. Although the underparts on tristis are cold white, the upperparts are actually quite a warm buffy colour, especially around the ear coverts. The rear of the supercilium is also quite buffy and there's often a slightly buff wash to the sides of the neck. When seen in the field, they completely lack any olive-green tones (they have green tones only on the underwing). Abietinus tend to have quite a bit of olive green on the tertials and coverts. There's photos of classic abietinus here and here. Note the steely grey tones contrasting with the olive-green in the wings. Tristis on the otherhand looks like this or this. Tristis also often have a very black bill and legs. Our collybitas can vary a bit in colour, but are generally much warmer than abietinus, usually have some olive-green tones, and are not nearly as pale underneath as either tristis or abietinus. See here and here
The Helston bird was calling. Tristis sound quite different. They have a monosyllibic call. Both abietinus and collybita give either the classic hueeet call (listen here) or a slightly more confusing pweet call (listen here), sometimes mistaken for "eastern" chiffchaff, but readily given by our collybitas. Most people seem to attribute this to the call of juvenile birds during Autumn migration. Oddly, I heard several giving this call last April and May. Some abietinus sound a bit more disyllabic, or almost trisyllabic (listen here). I heard a bird in East Norfolk give a call like this last November. To me tristis is a bit more sad than the others. It reminds me a bit of a Bullfinch (listen to the second "Siberische Tjiftjaf" down here). They can be quite variable though. Compare the other recording of the same bird.
Of course, there's also the complication of 'fulvescens' and other integrades. There are some more useful sites, which go into a lot more detail here, here, here and here.