I've had an email from David Jones Powell suggesting one reason for the disappearance of the Green Woodpecker, at least at a local level.
David says his ant-rich garden,near Brecon, was regularly visited by Green Woodpeckers until he saw one being killed by a Goshawk on his garden steps. He says the Gos was very bold and held its ground even when David and his son approached before flying off with the unfortunate pecker.
Now Goshawks nest there every year and there are no longer any Green Woodpeckers.
Of course, there are probably a number of factors involved, but it is interesting to hear a personal account. I remember hearing at the WOS conference years ago that Goshawks were suspected of being a factor in the decline of Kestrels in parts of mid-Wales.