The other problem with GSW is that it has white underparts. The Green WP has more uniform coloration underneath, though an adult one should appear distinctly colourful: a juvenile is rather grey-ish, but wouldn't appear dark Sacha gives the impression it was a fairly dark bird.

If it's definitely bigger than a thrush, what about a juvenile jackdaw? They have some paler patches around the head and neck. Or leucistic jackdaw?

My general approach is that on the whole a bird that is hard to identify is much more likely something fairly common misinterpreted than something rare. Most "suspected hoopoes" turn out to be jays, and most "suspected golden orioles" are green woodpeckers. Though of course on rare occasion they are as suspected. I have seen a smew on my local sewage works pond, a pair of Carolina wood ducks on the town pond, and a dalmatian pelican on a nearby reservoir, none of them were in any doubt. Though none of them is anything like as unlikely as a bird-book would have you beileve, because all of them are kept in collections and escape from time to time.