In case of Facebook flaws and stuff I've posted this here as well for you Craigy
I know you're worried about the tree not having enough canopy and your prob right, but in order to do this you need polys. So I give you permission to use whatever polygon count you need. In saying that just go a little crazy with the tree, take it into zbrush and snake hook branches or even just take the level put it into max and start filling up the atrium space with the tree. So to give you some inspiration, here you go.