What I do find is that the calls I hear of Greenish are often not easy to pigeonhole into disyllabic (the classic Greenish) or trisyllabic (typical of Green). Instead many seem to be more slurred and not easy to pick out individual syllables. There was a useful webpage with song and call recordings of both, which I think was on the Dutch Birding website but I can’t find it now (this isn’t the page but does have some useful recordings: http://www.dutchbirding.nl/journal.php?page_id=169). When I listened to these some time ago I was struck by how obvious the differentiation was, and how different to my experience of birds further east. I also remember reading a quote from Lars Svensson which suggested that adult Greenish may have a more disyllabic call than the more slurred call of first-winters.
Potentially the late date for the Church Cove bird could suggest a more easterly origin and, if the bird is a first-winter, confirming the id could be even trickier than more typical early autumn birds. Hope some good photos and sound recordings have been possible!
Record, share and compare with BUBO Listing at www.bubo.org