Hi Nigel,
The classic far East Tristis has no traces of yellow (other than the bend of the wing) or Olive unless on rump or tertials they have a distinct call and song. Fluvescents can have traces of yellow and olive in their plumage and have the same call and Song as eastern tristis.
The true grey and white Chiffchaff remains a mystery ( could be a result of integration) Martin Garner classes these as vey rare in Britain. Details explained on Alan Deans site
http://deanar.org.uk/tristis/tristis.htm with a case study. However Alan's paper are a few years old now and since genetic testing of Birds in recent Dutch and Cornish papers. Abietinus they don't know where the majority winter, some say fluvescent diagnosis needs widening. I'm afriad that it will take a few more years field work and genetic testing to sort these out. Then there may be a split with Siberian Chiffchaff as a separate species. Although Tristis and abietinus appear to hybridise, in comparison the Caspain Gull hybridises with Herring Gull and Yellow-legged Gull but has been split as a separate species.
At Ladywalk the matter to focus on, is to hear the Ringed Chiffchaff call, if it gives a Bullfinch type call this is a very good indicator.
Regards, John