Finally finished reading this - can't recommend it highly enough! Even if those bloody Green Horse Things from Project UFO were staring RIGHT AT ME for TWO WHOLE PAGES! Eeeek!
It also finally allowed me to identify the last of my half remembered childhood TV memories - 'a man, standing in a park at night, turns into a tree' - to an episode of 'Shadows' called 'The Man Who Hated Children' where George A Cooper from Grange Hill... turns into a tree. And it takes AGES. No wonder it stuck in my memory.