Siberian Chiffchaff?

about a week ago I heard what I assumed to be a tristis Chiffchaff at Alkborough but the only Chiffy I could see was a fairly typical looking collybita -- confusion -- this morning ND located two birds and in spite of the dense fog we had pretty good views and I managed some terrible images; the other bird not featured had a much stronger greater covert bar but neither would call which was frustrating; playback of eastern abietenus elicited no response but tristis calls from Hannu Jannes collection brought the bird straight to the phone looking inquisitively but still it would not call -- compared to the bird featured in the Birding World article by Anthony McGeehan it looks pretty good allowing for the terrible lighting -- no excuse for the terrible quality maybe get them in better light another day