Goodonya Paul
If they really can't part with the tree, maybe you could suggest to your wonderful and understanding neighbours that they should plant something else in it's place like a really pretty cherry blossom tree or something. That way they can still be "tree hugging hippies"
You can offer to buy it for them too as a present - bribery goes a long way...
I'm glad my dad got rid of our tree that was smack bang in the middle of our backyard years ago. The reason for the chop? Well, a branch kinda, sorta fell onto our neighbours fence during one freak thunderstorm... Missed my dad's shed though
I'd say that and the fire hazard excuse is a great incentive for any neighbour wanting to get rid of a tree
