I would carefully cut it back. Around here, it is a very large weed. I cut it down to 3 feet tall, and it grows back. The wind knocks them over, it grows back. I say carefully cut it because you wouldn't want to harm the greenhouse.

You might want to involve the neighbour in this exercise... if the tree damages the greenhouse you will be liable.

Cut it, as the others say, carefully so as not to damage the greenhouse. Unless you want an 80 odd foot tree in your garden, you ought to cut it yearly anyway - average growth is 8 feet a year.